Correctly serialize non ASCII metadata

[lukaszsamson]

This PR fixes serialization of non ASCII topic metadata. It also adds assert on byte size matchin asserts on server side in lib/phoenix/socket/serializers/v2_json_serializer.ex which limit fields to 255 bytes

Fixes #6663

With this patch Repro https://github.com/lukaszsamson/phoenix-serializer-bug-repro works correctly:

Server:

[info] CONNECTED TO CtfWeb.UserSocket in 27µs
  Transport: :websocket
  Serializer: Phoenix.Socket.V2.JSONSerializer
  Parameters: %{"vsn" => "2.0.0"}
[info] JOINED echo:room:café in 26µs
  Parameters: %{}
[debug] HANDLED echo_bin INCOMING ON echo:room:café (CtfWeb.EchoChannel) in 4µs
  Parameters: {:binary, "π-data-🚀"}

Client:

window.runRoundtrip()
Promise {<pending>}
frame:1829 📡 [info] CONNECTED TO CtfWeb.UserSocket in 14µs  Transport: :websocket  Serializer: Phoenix.Socket.V2.JSONSerializer  Parameters: %{"vsn" => "2.0.0"}
frame:1829 📡 [info] JOINED echo:room:café in 12µs  Parameters: %{}