Handles implicit rendering for a controller action that does not explicitly
respond with render, respond_to,
redirect, or head.
For API controllers, the implicit response is always
204 No Content.
For all other controllers, we use these heuristics to decide whether to
render a template, raise an error for a missing template, or respond with
204 No Content:
First, if we DO find a template, it's rendered. Template lookup
accounts for the action name, locales, format, variant, template handlers,
and more (see render for details).
Second, if we DON'T find a template but the controller action does have
templates for other formats, variants, etc., then we trust that you meant
to provide a template for this response, too, and we raise
ActionController::UnknownFormat with an explanation.
Third, if we DON'T find a template AND the request is a page load in a
web browser (technically, a non-XHR GET request for an HTML response) where
you reasonably expect to have rendered a template, then we raise
ActionView::UnknownFormat with an explanation.
Finally, if we DON'T find a template AND the request isn't a
browser page load, then we implicitly respond with 204 No
Content.