Support json payload body

[seppo0010]

To add alt text to an image the request must be a json object:

https://developer.twitter.com/en/docs/twitter-api/v1/media/upload-media/api-reference/post-media-metadata-create

I think a small tweak in the __request method could do it, but I'm not sure

https://github.com/desmondmorris/node-twitter/blob/296e82ede50ddace34ad64952fdfacfe8b1c65fc/lib/twitter.js#L136-L143