Speed up view specs by re-enabling resolver cache

The caching was turned off in [1] because different formats were being
cached under the same key. I believe the need to do this was removed
in the previous commit.

This reduces the time it takes to run FutureLearn’s view specs by 50%.

I think it’s sensible to use the view resolver cache during tests because
it’s normally on between requests in other environments including when
creating tests with `ActionView::TestCase` in a new Rails application.
It looks like it was only turned off in rspec-rails to work around this
caching issue.

I haven't added any test coverage in this change since it feels like
it would be testing a negative (the resolver cache isn't cleared).

[1] rspec#864

Author: zetter

lib/rspec/rails/example/view_example_group.rb

@@ -155,9 +155,6 @@ def _include_controller_helpers
             view.lookup_context.prefixes << _controller_path
           end
 
-          # fixes bug with differing formats
-          view.lookup_context.view_paths.each(&:clear_cache)
-
           controller.controller_path = _controller_path
           controller.request.path_parameters[:controller] = _controller_path
           controller.request.path_parameters[:action]     = _inferred_action unless _inferred_action =~ /^_/