Compare host scheme using case-insensitive regexp

Before:

    image_tag("HTTP://google.com")
    # => "<img alt=\"Google\" src=\"/assets/HTTP://google.com\" />"
    image_tag("http://google.com")
    # => "<img alt=\"Google\" src=\"http://google.com\" />"

After:

    image_tag("HTTP://google.com")
    # => "<img alt=\"Google\" src=\"HTTP://google.com\" />"
    image_tag("http://google.com")
    # => "<img alt=\"Google\" src=\"http://google.com\" />"

Backport of #10969
indicate the default Rails cache store
memory_store is the default cache store implementation if you call
ActiveSupport::Cache.lookup_store without arguments.
Merge pull request #10883 from ifeelgoods/3-2-stable

The default cache store is :file_store in caching_with_rails.textile