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

node-twitter/lib/twitter.js

Lines 136 to 143 in 296e82e

	if (method === 'post') {
	var formKey = 'form';
	if (typeof params.media !== 'undefined') {
	formKey = 'formData';
	}
	options[formKey] = params;
	}