Fix minor regression with Auth feature and REST/Memory backend

See https://matrix.to/#/!NPRUEisLjcaMtHIzDr:kamax.io/$1523216730848820dFUZX:matrix.org

src/main/java/io/kamax/mxisd/backend/memory/MemoryIdentityStore.java

@@ -163,7 +163,7 @@ public class MemoryIdentityStore implements AuthenticatorProvider, IDirectoryPro
                 return BackendAuthResult.failure();
             } else {
                 BackendAuthResult result = new BackendAuthResult();
-                id.getThreepids().forEach(result::withThreePid);
+                id.getThreepids().forEach(tpid -> result.withThreePid(new ThreePid(tpid.getMedium(), tpid.getAddress())));
                 result.succeed(mxid.getId(), UserIdType.MatrixID.getId(), "");
                 return result;
             }