integration test support for the options request method

this adds support, in integration tests (ActionDispatch::IntegrationTest), to call "options" similar to the currently supported get, post, head.. and the rest. ie:

    def test_options_for_search
      options '/search'
      assert_response :success
      # etc
    end


options is a real enough request method, and it is clumsy to test it without this added method.

re: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html

i imagine that "options" is a tricky thing to name a method, but i don't see any conflicts near this space.

let me know and i'll CHANGELOG.