fix: correct plugin validation code (#81)

The plugin validation endpoint checked if the returned status was 201, 
whereas upstream at 
https://github.com/Kong/kong/blame/f95f755864589ff2884aa7f4f6f3da7e40063
980/kong/api/routes/kong.lua#L157 the endpoint returns a 200. Checking 
against 201 appears to be a hidden error in the original 
implementation, which de facto only failed on errors. 201s are still 
allowed because it's unclear if there was some reason for that check 
originally, and allowing 201s is benign (actual failures will return a 
400).

kong/plugin_service.go

@@ -154,7 +154,7 @@ func (s *PluginService) Validate(ctx context.Context, plugin *Plugin) (bool, err
 	if err != nil {
 		return false, err
 	}
-	return resp.StatusCode == http.StatusCreated, nil
+	return resp.StatusCode == http.StatusCreated || resp.StatusCode == http.StatusOK, nil
 }
 
 // listByPath fetches a list of Plugins in Kong