Let testing compile

Author: mbax

src/test/java/org/kitteh/irc/client/library/util/StubServerInfo.java

@@ -25,6 +25,11 @@ public void addCustomChannelMode(@NonNull ChannelMode mode) {
 
     }
 
+    @Override
+    public void addCustomUserMode(@NonNull UserMode mode) {
+
+    }
+
     @Override
     public @NonNull Optional<String> getAddress() {
         return Optional.empty();