Fix new 3PID bindings being associated with rewritten IS's server nam…

…es instead of the original one

This would make unbind requests fail because Synapse is using the server name it has in DB as destination_is to generate signature, and Sydent uses its own server name there when verifying it, so if a mismatch happens, which is the case if the name gets rewritten for routing purposes, the unbind requests fail on the signature verification.

synapse/handlers/identity.py

@@ -129,14 +129,19 @@ def bind_threepid(self, creds, mxid):
             client_secret = creds['clientSecret']
         else:
             raise SynapseError(400, "No client_secret in creds")
+
         # if we have a rewrite rule set for the identity server,
-        # apply it now.
+        # apply it now, but only for sending the request (not
+        # storing in the database).
         if id_server in self.rewrite_identity_server_urls:
-            id_server = self.rewrite_identity_server_urls[id_server]
+            id_server_host = self.rewrite_identity_server_urls[id_server]
+        else:
+            id_server_host = id_server
+
         try:
             data = yield self.http_client.post_urlencoded_get_json(
                 "https://%s%s" % (
-                    id_server, "/_matrix/identity/api/v1/3pid/bind"
+                    id_server_host, "/_matrix/identity/api/v1/3pid/bind"
                 ),
                 {
                     'sid': creds['sid'],