Assign new ResponseWriter after calling http.HandlerFunc (#1341)

Otherwise, the `http.ResponseWriter` passed to `next()` within the
middleware is unused. This precludes middlewares from wrapping the
http.ResponseWriter to do things like record the status code.

echo.go

@@ -767,6 +767,7 @@ func WrapMiddleware(m func(http.Handler) http.Handler) MiddlewareFunc {
 		return func(c Context) (err error) {
 			m(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 				c.SetRequest(r)
+				c.SetResponse(NewResponse(w, c.Echo()))
 				err = next(c)
 			})).ServeHTTP(c.Response(), c.Request())
 			return