Send 3PID bind requests as JSON data

changelog.d/5656.bugfix

+Fix 3PID bind requests being sent to identity servers as `application/x-form-www-urlencoded` data, which is deprecated.

synapse/handlers/identity.py

@@ -118,7 +118,7 @@ class IdentityHandler(BaseHandler):
             raise SynapseError(400, "No client_secret in creds")
 
         try:
-            data = yield self.http_client.post_urlencoded_get_json(
+            data = yield self.http_client.post_post_get_json(
                 "https://%s%s" % (id_server, "/_matrix/identity/api/v1/3pid/bind"),
                 {"sid": creds["sid"], "client_secret": client_secret, "mxid": mxid},
             )