authenticating with API key is broken; request overwrites X-Redmine-Api-Key header from BaseEngine.__init__ #353
Description
The fairly-new construct_request_kwargs() takes a param "headers" which it uses exclusively to create the request headers; however, if the engine object was instantiated with 'key', the key is stored in self.requests['headers'] already, and should be merged in construct_request_kwargs(), not overwritten