Split the 1200+ line mime_responds_test into 3 more focused and manageable test files.
Fail informatively in #respond_with when no appropriate #api_behavior renderer is available.

Currently if a user calls #respond_with(csvable), but has not csv
renderer available, Responder will just run through the default render
behavior twice, raising ActionView::MissingTemplate both times.

This changes ActionController::Metal::Responder#api_behavior to check
in advance whether there is a renderer available, and raise
ActionController::MissingRenderer if not.
Merge pull request #11926 from Empact/respond-with-csv

Fail informatively in #respond_with when no appropriate #api_behavior renderer is available.