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
 }