Modified default attribute error message generation

Generation of the default attribute error message is now delegated to
ActiveModel::Errors where possible, avoiding inconsistencies where
default locale entries have been overridden from the ActiveModel
defaults and other divergent behavior.
Refactored delegation of error message generation

Moved delegation to ActiveModel::Errors#generate_message to
default_error_message and implemented a spec for it.
Fixed typo in spec for error message generation
Refactored error-generation spec to use inline fixture