Copy headers set in request handlers to main handler

When you re-implemented my previous pull request for setting the Content-Type header, you left out the most critical part. The header is set on a different instance of the request handler, so when the request is finished (on the main handler), the new headers are never seen.

You can test this by doing using wget to request a poster and viewing the headers output by sickrage. They show "text/html" instead of the correct "image/jpeg".

This pull request contains the 3 new lines of code to copy the headers from the request handler where they're set into the main request handler. There may be a better way to do this, but this works.