Synapse queries the IS's /store-invite endpoint with the wrong content-type

[babolivier]

Synapse currently sends requests to /store-invite using the application/x-www-form-urlencoded content-type header: https://github.com/matrix-org/synapse/blob/master/synapse/handlers/room_member.py#L908

The IS spec says:

all API calls use a Content-Type of application/json

It currently works because Sydent supports both application/x-www-form-urlencoded and application/json but this doesn't sound like something we'd want to do.

[babolivier]

Fixed on develop