Code review comment for lp:~leonardr/lazr.restful/multiversion-pagetest

This branch adds more multi-version capabilities to the example multiversion web service (fields that are not published in all versions, a field that's manipulated with a mutator, and two destructor methods for different versions), and adds pagetests to make sure all the features I've been writing work in a real multi-version web service.

I found one part of the system which did _not_ work in a real web service. If you define a field's mutator method for version 1.0, that works fine for version 1.0, but the mutator is not inherited in version 2.0--the field goes back to being read-only. There was no inheritance mechanism to make sure that version 2.0's mutator defaults to version 1.0's mutator.

Fortunately, I've already dealt with this situation. I had the exact same problem on the field level, where version 2.0's field was not published under version 1.0's name, and I resolved it with _normalize_field_annotations. In this branch, I removed the _get_by_version helper method which I was using to retrieve mutator information from the special mutator dictionary. Instead, I changed _normalize_field_annotations to merge the special mutator dictionary into the standard annotation dictionary as it normalizes that dictionary. This is where inheritance is implemented for fields, and now that's also where it's implemented for field mutators.