Add "setHeader" to add/change headers on websocket client

[tyagoy]

I am developing a project where each time the connection is closed, I generate a new token and add the header to the client using the "addHeader" API. The problem is that this API is not suitable for modifying headers; this method performs a "_customHeaders.push_back". If a specific header needs to be changed, there is no API to do that. Moreover, this "push_back" might start accumulating memory if called multiple times. I believe it would be good to have a new "setHeader" method that adds and/or modifies a header in "_customHeaders".

[aqexhu]

Created the setHeader, which accomplishes your request. Feel free to use it. https://github.com/gilmaimon/ArduinoWebsockets/pull/172