[PATCH] DateTime#to_date returns self when Date#to_date is defined at first

On Ruby 1.8.7 DateTime#to_date returns self due to the following bug.

As active_support/time.rb loads active_support/core_ext/date/conversions before active_support/core_ext/date_time/conversions then DateTime#to_date method is not defined as DateTime.method_defined?(:to_date) is true as Date#to_date method is already defined and DateTime inherits to_date method from Date.

There is similar issue with DateTime#xmlschema method definition.

Included patch fixes conditions for DateTime#to_date and DateTime#xmlschema definition to check if method is defined for DateTime and is not just inherited. Included patch also removes requiring 'active_support/ruby/shim' in abstract_unit.rb when using Ruby 1.8.7 to ensure the same sequence of requires as in real Rails applications.