Fix bug when querying remote user keys that require a resync. (#6796)

We ended up only returning a single device, rather than all of them.

changelog.d/6796.bugfix

@@ -0,0 +1 @@
+Fix bug where querying a remote user's device keys that weren't cached resulted in only returning a single device.

synapse/handlers/e2e_keys.py

@@ -208,8 +208,9 @@ class E2eKeysHandler(object):
                         )
 
                     user_devices = user_devices["devices"]
+                    user_results = results.setdefault(user_id, {})
                     for device in user_devices:
-                        results[user_id] = {device["device_id"]: device["keys"]}
+                        user_results[device["device_id"]] = device["keys"]
                     user_ids_updated.append(user_id)
                 except Exception as e:
                     failures[destination] = _exception_to_failure(e)