Include fields that have attribute transformers.

Previously, only attributes that didn't match fields, but had attribute
transformers were included in attributes from proto. This omitted any
fields that had transformers, but don't have actual attributes (e.g.
attr_accessor). Now any fields with attribute transformers are included.
Update spec support for attr transformers w/out attrs
Update specs to handle fields w/out attributes.
Bump version to 3.0.1.