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.

context.go

@@ -26,6 +26,9 @@ type (
 		// SetRequest sets `*http.Request`.
 		SetRequest(r *http.Request)
 
+		// SetResponse sets `*Response`.
+		SetResponse(r *Response)
+
 		// Response returns `*Response`.
 		Response() *Response
 
@@ -228,6 +231,10 @@ func (c *context) Response() *Response {
 	return c.response
 }
 
+func (c *context) SetResponse(r *Response) {
+	c.response = r
+}
+
 func (c *context) IsTLS() bool {
 	return c.request.TLS != nil
 }

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