Renderer refactors

Some small refactors to Action View renderers:

- Marked methods private which don't work on their own anyways (e.g. in `PartialRenderer`, only the main `render` method sets up instance vars which are needed for the other render methods to work).
- Added some missing test coverage
- Simple line-level code cleanup

Leaving commits separate for review, can squash before merge if approved.

I think more substantial refactors can be done to the renderers, but want to start with the simpler stuff first.