Support string

[softmarshmallow]

AWS websocket does not allow us to use byte[] for the data input. It just simply is impossible. Clearly, it's poor design from aws, but anyhow we'll definitely need string support for this package, since socketsharp does.

ref:

• https://docs.aws.amazon.com/apigateway/latest/developerguide/websocket-api-develop-binary-media-types.html

[softmarshmallow]

Using https://github.com/endel/NativeWebSocket/