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