Doesn't create correct bounds for optional RefMut parameters

[sdroege]

They end up Into<&'a Bla> instead of Into<&'a mut Bla>. Example here: https://github.com/sdroege/gstreamer-rs/blob/50a1535771d711b419eee499fb6b7c48d72edd86/gstreamer/src/functions.rs#L18

When generating that automatically, it ends up without the mut and also let context = context.into() instead of let mut context = context.into()

[EPashkin]

For me by documentation this case (and most others) just don't need mut at all as it Boxed, so just const=true for this parameter.

[EPashkin]

OOps, mistaken Boxed and Shared, now not fully sure for this case.

[sdroege]

For Boxed it should be mut, and without Option it also does that correctly already. bound_rust_type() seems to forget that information, the RefMode.