Fix rake templates:test

[mishina2228]

rake templates:test was left to fail for a long time.
This PR will fix it (with a slightly larger number of file changes).
Here are the details:

• Fix for the edge Rails
  • Address the change of ActionView::Base.new in Rails 6
    • Same change as Instantiate view with empty template cache #304
  • Render templates without . in the name
    • Similar change as Prevent deprecation warnings by removing template formats #298
    • refs:
      • Deprecate rendering templates with . in the name rails#39164
      • https://stackoverflow.com/questions/68836670/rails-6-1-4-deprecation-warning-rendering-actions-with
  • Check Rails.logger is defined
    • ActionView is in use without Rails at rake templates:test
  • require "action_dispatch" to use Mime::Type
    • This fixes the following error:

      ActionView::Template::Error: undefined method `to_sym' for nil:NilClass
      
                @symbol = symbol.to_sym
                          ^^^^^^^
      

• Fix for the edge Rack
  • Lowercase response headers
    • The latest rack 3.0 (unreleased) does not accept uppercase headers.
      See Make response headers a hash and enforce lower-case keys. rack/rack#1592 for more details.
    • refs:
      • Change SPEC to not allow uppercase header keys rack/rack#1812
      • Lowercase all response headers sprockets#744
    • It required changes not only to files under test/, also to files under lib/.
  • Set content-length header in the FakeMiddleware
    • Similar change as Fix Content-Length for rack >= 2.1.0 #290
• Replace mocha-phantomjs-core with mocha-headless-chrome
  • web-console has ES6 syntax codes introduced in Use MutationObserver instead of Mutation Events #297 and Fix js string error - closes #240 #308.
  • phantomjs does not support ES6, so it is replaced with mocha-headless-chrome.

I ran the test locally and confirmed that it succeeded.

$ bundle exec rake templates:test


..............................
30 passing (679ms)

[gsamokovarov]

Thanks for all the little details you fixed along the way! 🙇‍♂️