Failing test case: Enable "active_support/string/conversions"

Description: ActiveSupport string conversion adds a `to_date` method to *all* string objects. And when called, raises an exception when the date is invalid. So merely checking respond_to?(:to_date) is not enough. We need to ignore the invalid cases by considering them as "false" (could not convert)
Merge pull request #15 from swanandp/fix_for_active_support_conversions

Failing test case: Enable "active_support/string/conversions"