remove content headers for 303 redirect

when doing a GET request for a 303 redirect clear the Content-Length
and Content-Type headers, they are set during the initial POST.

tornado/simple_httpclient.py

@@ -361,6 +361,11 @@ class _HTTPConnection(object):
             if self.code == 303:
                 new_request.method = "GET"
                 new_request.body = None
+                for h in ["Content-Length", "Content-Type"]:
+                    try:
+                        del self.request.headers[h]
+                    except KeyError:
+                        pass
             new_request.original_request = original_request
             final_callback = self.final_callback
             self.final_callback = None