Use &encoding for msgpack Packer

- This commit disables automatic encoding of Unicode strings until
  &encoding is retrieved, and then resets the packer to use &encoding
- Previously the msgpack Packer always encoded Unicode as utf-8

I have not checked, but I think this was working previously and regressed. Basically this example should use &encoding instead of utf8

```
vim.current.line = u'¬'
```

This follows from #41. But it is not python3 specific.
I think you need to add a `.` here for flake8:

`neovim/msgpack_rpc/msgpack_stream.py:29:1: D400  First line should end with '.', not 's'`