Ensure responses are context.ResponseWriters (#19843)

In order for web.Wrap to be able to detect if a response has been written
we need to wrap any non-context.ResponseWriters as a such. Otherwise
responses will be incorrectly detected as non-written to and handlers can
double run.

In the case of GZip this handler will change the response to a non-context.RW
and this failure to correctly detect response writing causes fallthrough and
a NPE.

Fix #19839

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/web/wrap_convert.go

@@ -21,6 +21,9 @@ func convertHandler(handler interface{}) wrappedHandlerFunc {
 	case http.HandlerFunc:
 		return func(resp http.ResponseWriter, req *http.Request, others ...wrappedHandlerFunc) (done bool, deferrable func()) {
 			routing.UpdateFuncInfo(req.Context(), funcInfo)
+			if _, ok := resp.(context.ResponseWriter); !ok {
+				resp = context.NewResponse(resp)
+			}
 			t(resp, req)
 			if r, ok := resp.(context.ResponseWriter); ok && r.Status() > 0 {
 				done = true
@@ -92,6 +95,9 @@ func convertHandler(handler interface{}) wrappedHandlerFunc {
 				next = wrapInternal(others)
 			}
 			routing.UpdateFuncInfo(req.Context(), funcInfo)
+			if _, ok := resp.(context.ResponseWriter); !ok {
+				resp = context.NewResponse(resp)
+			}
 			t(next).ServeHTTP(resp, req)
 			if r, ok := resp.(context.ResponseWriter); ok && r.Status() > 0 {
 				done = true