[PATCH] Fix JSON serialization of ActiveModel::Errors

JSON encoding an ActiveModel::Errors object with :only option currently breaks:

>> ActiveSupport::JSON.encode(ActiveModel::Errors.new(nil), :only => :attribute)
ArgumentError: wrong number of arguments (0 for 1)
from /Users/sam/Development/rails/activesupport/lib/active_support/core_ext/hash/slice.rb:17:in `initialize'
from /Users/sam/Development/rails/activesupport/lib/active_support/core_ext/hash/slice.rb:17:in `new'
from /Users/sam/Development/rails/activesupport/lib/active_support/core_ext/hash/slice.rb:17:in `slice'
from /Users/sam/Development/rails/activesupport/lib/active_support/json/encoding.rb:210:in `as_json'
from /Users/sam/Development/rails/activesupport/lib/active_support/json/encoding.rb:48:in `encode'
from /Users/sam/Development/rails/activesupport/lib/active_support/json/encoding.rb:62:in `check_for_circular_references'
from /Users/sam/Development/rails/activesupport/lib/active_support/json/encoding.rb:47:in `encode'
from /Users/sam/Development/rails/activesupport/lib/active_support/json/encoding.rb:32:in `encode'
from (irb):2

This fails as Hash#slice in ActiveSupport tries to instantiate a new ActiveModel::Errors with no arguments (one arg is required).

This patch changes the behavior of Hash#slice to dup and clear the hash, which will work even when hash subclasses change the #initialize parameters.