Use assert_kind_of whenever possible instead of assert

Attached is patch.

Is ok but do the same i said on #4644 Use assert_respond_to because it has better error messaging

$ grep -r 'assert ' * | grep is_a

and

$ grep -r 'assert ' * | grep kind_of

i get the following ...

actionpack/test/controller/new_base/bare_metal_test.rb: assert part.is_a?(String), "Each part of the body must be a String"
activemodel/lib/active_model/lint.rb: assert model.errors[:hello].is_a?(Array), "errors#[] should return an Array"
activemodel/lib/active_model/lint.rb: assert model.errors.full_messages.is_a?(Array), "errors#full_messages should return an Array"
activerecord/test/cases/query_cache_test.rb: assert Task.connection.select_value("SELECT count(*) AS count_all FROM tasks").is_a?(Numeric)
activerecord/test/cases/fixtures_test.rb: assert fixture.is_a?(Fixtures)
activesupport/test/safe_buffer_test.rb: assert @buffer.is_a?(String)
activesupport/test/core_ext/time_with_zone_test.rb: assert result.is_a?(Float)
activesupport/test/core_ext/time_with_zone_test.rb: assert result.is_a?(Integer)
activesupport/test/core_ext/time_with_zone_test.rb: assert @twz.is_a?(Time)
activesupport/test/core_ext/time_with_zone_test.rb: assert @twz.is_a?(ActiveSupport::TimeWithZone)
activesupport/test/core_ext/duration_test.rb: assert d.is_a?(ActiveSupport::Duration)
activesupport/test/core_ext/duration_test.rb: assert d.is_a?(Numeric)
activesupport/test/core_ext/duration_test.rb: assert d.is_a?(Fixnum)
activesupport/test/core_ext/duration_test.rb: assert !d.is_a?(Hash)
activesupport/test/core_ext/duration_test.rb: assert !d.is_a?(k)
activesupport/test/core_ext/date_time_ext_test.rb: assert DateTime.current.is_a?(DateTime)
activesupport/test/core_ext/date_time_ext_test.rb: assert DateTime.current.is_a?(DateTime)
railties/test/application/rackup_test.rb: assert Rails.application.is_a?(Rails::Application)
railties/test/application/rackup_test.rb: assert ActionController::Dispatcher.new.is_a?(Rails::Application)
activerecord/test/cases/inheritance_test.rb: assert Company.find(1).kind_of?(Firm), "37signals should be a firm"
activerecord/test/cases/inheritance_test.rb: assert Firm.find(1).kind_of?(Firm), "37signals should be a firm"
activerecord/test/cases/inheritance_test.rb: assert Company.find(2).kind_of?(Client), "Summit should be a client"
activerecord/test/cases/inheritance_test.rb: assert Client.find(2).kind_of?(Client), "Summit should be a client"
activerecord/test/cases/inheritance_test.rb: assert companies[0].kind_of?(Firm), "37signals should be a firm"
activerecord/test/cases/inheritance_test.rb: assert companies[1].kind_of?(Client), "Summit should be a client"
activerecord/test/cases/inheritance_test.rb: assert next_angle.kind_of?(Firm), "Next Angle should be a firm"
activesupport/test/core_ext/time_with_zone_test.rb: assert @twz.kind_of?(Time)
activesupport/test/core_ext/string_ext_test.rb: assert UNICODE_STRING.mb_chars.kind_of?(String)
activesupport/test/core_ext/string_ext_test.rb: assert UNICODE_STRING.mb_chars.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/core_ext/string_ext_test.rb: assert UNICODE_STRING.mb_chars.kind_of?(String)
activesupport/test/multibyte_chars_test.rb: assert @chars.method_for_multibyte_testing.kind_of?(@proxy_class)
activesupport/test/multibyte_chars_test.rb: assert @chars.method_for_multibyte_testing!.kind_of?(@proxy_class)
activesupport/test/multibyte_chars_test.rb: assert character.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').insert(0, '').kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').rjust(1).kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').ljust(1).kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').center(1).kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').rstrip.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').lstrip.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').strip.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').reverse.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars(' ').slice(0).kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').limit(0).kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').upcase.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').downcase.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').capitalize.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').normalize.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').decompose.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').compose.kind_of?(ActiveSupport::Multibyte.proxy_class)
activesupport/test/multibyte_chars_test.rb: assert chars('').tidy_bytes.kind_of?(ActiveSupport::Multibyte.proxy_class)

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