Follow meta refreshes for responses that only contain a refresh meta tag (e.g. t.co responses)

Mechanize currently doesn't follow t.co meta refreshes because they have a response without a `<head>`. For example, the response for [http://t.co/Lizw8srs](http://t.co/Lizw8srs) is simply:

    <noscript><META http-equiv="refresh" content="0;URL=http://bit.ly/XFwn3X"></noscript><script>location.replace("http:\/\/bit.ly\/XFwn3X")</script>

The ['head > meta' selector](https://github.com/sparklemotion/mechanize/blob/master/lib/mechanize/page.rb#L374) doesn't match this, so Mechanize doesn't follow this redirect.

It looks like the original intent of `agent.follow_meta_refresh = :anywhere` was to avoid following meta refreshes outside of `<head>` (see #13 & #99), but this seems to be an entirely different case.

These `<meta>`-only responses are explicit in intent, and fetching t.co URLs is likely a very common need.