-protoset-out not working when invoking RPC

The documentation for `-protoset-out` includes:

```
When invoking an RPC and this option is given, the method being invoked and its transitive dependencies will be included in the output file.
```

However, when I attempt to use the option when invoking an RPC no file in written. Looking at the current code, it appears the function to write the protoset is not called within the invoke if block.

For my use case, ideally the `-protoset-out` would work as documented. If it was decided to not work with invoking, I think the documentation needs to be updated and a validation error added.