Correctly set error response content type.
Make send_error populates Core::Error with apps serializer.

Core::Error has support for a serializer as a read-only attribute. Ensure
this is populared when the Error object is created if configured for the
app.
Serialize error message if a serializer is defined in Error object. Ref #209.

Default error content is the serialized hash of the message, title,
status and exception if a serializer exists.

Default error content_type is set from the serializers content_type.
Update tests for error serialization.

Check response content type and (correct) response body.
Hooks tests updated to work with either an array or hash ref.
Changes requested by xsawyerx.
Update changes
Merge branch 'veryrusty-Pr/error_serialization'