remove 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.
Merge pull request #57 from bennofs/redirect-reset-header

clear Content-Type header on 302/303 redirect