clear Content-Type header on 302/303 redirect

Prior to this change, when doing a POST multipart request that returns a
redirect, the redirected request still had the multipart/form-data content 
type, even though it's a GET request. Now, we just don't set a Content-Type
when following a redirect with a GET request.

I'm currently trying to interface with a site that returns a 500 Internal Server Error when
the GET request has a multipart content type. Chrome also clears the Content-Type header.

Maybe there are also other headers which should be cleared?