Duplicate custom headers

[FlorianCassayre]

A quick inspection in Wireshark revealed that the custom headers appeared twice in the request.

This sample code

val client = WebsocketClient(uri, protocolHandler, Map("test" -> "value"))
val ws = client.open()

produces the following request

GET / HTTP/1.1
upgrade: websocket
connection: upgrade
sec-websocket-key: [...]
host: [...]
sec-websocket-origin: [...]
sec-websocket-version: 13
test: value
test: value