fix cache incorrect http response code (#36)

cache.go

@@ -328,11 +328,9 @@ func (e *Entry) writePublicResponse(w http.ResponseWriter) error {
 
 func (e *Entry) writePrivateResponse(w http.ResponseWriter) error {
 	// wrap the original response writer
+	w.WriteHeader(e.Response.Code)
 	e.Response.SetBody(backends.WrapResponseWriterToBackend(w))
 	e.Response.WaitClose()
-	if !e.Response.IsFirstByteWritten {
-		w.WriteHeader(e.Response.Code)
-	}
 	return nil
 }
 

cache_test.go

@@ -187,6 +187,7 @@ func (suite *EntryTestSuite) TestEntryWritePrivateResponse() {
 	entry := NewEntry("unique_key2", req, res, suite.config)
 
 	go func() {
+		entry.Response.Write([]byte(`Bad Gateway`))
 		entry.Response.Close()
 	}()
 

response.go

@@ -79,7 +79,7 @@ func (r *Response) writeHeader(b []byte, str string) {
 		h.Set("Content-Type", http.DetectContentType(b))
 	}
 
-	r.WriteHeader(200)
+	r.WriteHeader(r.Code)
 }
 
 // Write writes the upstream's content in the backend's storage