[3.4.2] register_method does not accept arguments passed by reference

Let suppose a member method like this:

```
void Foo::bar(godot::String const name);
```
If we want to make it uses for GDNative script, we have to export it:

```
godot::register_method("bar", &Foo::bar);
```

This makes pass the string by copy and this works fine ! But now let pass `name` by reference:

```
void Foo::bar(godot::String const& name);
```

But this reference will produce a warning:
```
...

/home/qq/godot/3.4.2-stable/cpp/include/core/Godot.hpp:163:10: warning: returning reference to temporary [-Wreturn-local-addr]
163 | return a;
| ^
```

And possibly gives a crash. So How to deal: passing C++ reference with register_method ?

PS: register_method and C++ references worked well within Godot modules (C++ code inside the Godot editor code source inside the modules/ folder). I had no warnings.