Add check for legality of the returned Matrix ID in Auth

- Helps troubleshoot reported issues that might not be obvious at first
- Add basic unit test for auth manager

src/main/java/io/kamax/mxisd/auth/AuthManager.java

@@ -64,6 +64,8 @@ import java.util.Objects;
 
 public class AuthManager {
 
+    private static final Logger log = LoggerFactory.getLogger(AuthManager.class);
+
     private static final String TypeKey = "type";
     private static final String UserKey = "user";
     private static final String IdentifierKey = "identifier";
@@ -72,7 +74,6 @@ public class AuthManager {
     private static final String UserIdTypeValue = "m.id.user";
     private static final String ThreepidTypeValue = "m.id.thirdparty";
 
-    private transient final Logger log = LoggerFactory.getLogger(AuthManager.class);
     private final Gson gson = GsonUtil.get(); // FIXME replace
 
     private List<AuthenticatorProvider> providers;
@@ -138,6 +139,12 @@ public class AuthManager {
                     invMgr.publishMappingIfInvited(new ThreePidMapping(pid, mxId));
                 }
 
+                try {
+                    MatrixID.asValid(mxId);
+                } catch (IllegalArgumentException e) {
+                    log.warn("The returned User ID {} is not a valid Matrix ID. Login might fail at the Homeserver level", mxId);
+                }
+
                 invMgr.lookupMappingsForInvites();
 
                 return authResult;