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.