encoding implemeted incorrectly

[milyin]

withSchema method should append modificator after ;, not replace the whole encoding string.

To do:

• verify how it's done in Rust
• fix
• make corresponding test

[milyin]

Todo: test also that encoding is correctly passed through the e.g. pub/sub.
Seems like this should be fixed on the Rust protocol part first: I expect problems with custom encodings now.
So not closing the issue for now