Fix content_tag_for with array :class option

It would embed array as string instead of joining it like `content_tag` does:

    content_tag(:td, class: ["foo", "bar"]){}
    #=> '<td class="foo bar"></td>'

Before:

    content_tag_for(:td, item, class: ["foo", "bar"]){}
    #=> '<td class="item [&quot;foo&quot;, &quot;bar&quot;]" id="item_1"></td>'

After:

    content_tag_for(:td, item, class: ["foo", "bar"]){}
    #=> '<td class="item foo bar" id="item_1"></td>'

Fix to databases.rake so third party adapters can register rake task implementations

Remove pre-emptive adapter name check from databases.rake so third party adapters can register rake tasks

extensions

Remove yaml Proc param parser test

Minitest has changed to require this since https://github.com/seattlerb/minitest/commit/728054b

I don't believe this test is exercising any explicit params_parser
behavior that the other two Proc tests aren't already doing.  Given
that we now know it's a bad idea to load user input via YAML.load,
somebody reading this test might get a dangerous idea about building
out a YAML params parser.

Fix code comment

[ci skip]

Improve String#squish whitespaces matching

Make current_url? work with HEAD requests and remove caching_allowed? since it doesn't do anything

ActionDispatch::Head was removed in favor of Rack::Head. But Rack::Head
does not convert GET requests to HEAD requests so we need to do
checking for HEAD requests ourselves.

Where is this used? No other code references to this method and it
isn't being tested anywhere. No tests fail when commented out.