Make v2_alpha reg follow the AS API specification

The spec is clear the key should be 'user' not 'username' and this is indeed
the case for v1. This is not true for v2_alpha though, which is what this
commit is fixing.

synapse/rest/client/v2_alpha/register.py

@@ -100,6 +100,11 @@ class RegisterRestServlet(RestServlet):
 
         # == Application Service Registration ==
         if appservice:
+            # Set the desired user according to the AS API (which uses the
+            # 'user' key not 'username'). Since this is a new addition, we'll
+            # fallback to 'username' if they gave one.
+            if isinstance(body.get("user"), basestring):
+                desired_username = body["user"]
             result = yield self._do_appservice_registration(
                 desired_username, request.args["access_token"][0]
             )