Append new headers to the client when reconnecting

[cyyynthia]

Is your feature request related to a problem? Please describe.
I'm developing a client for an app that accepts a Resume-Id header when reconnecting for resuming a session. For now it looks like it's impossible to alter the list of headers which is a requirement for that kind of stuff

Describe the solution you'd like
Allow headers to be added, maybe only when socket is not connected.

Describe alternatives you've considered
It can be a .addHeader function, or a hook "onBeforeConnect" to add headers there

[haruntuncay]

If this is planned as a new-feature or as an enhancement, I would like to help.

[marci4]

Maybe simple adding a setter would be enough?

[haruntuncay]

We could add a method to put additional headers and it could initialize private Map<String,String> headers; variable, if not initialized yet, and then just add the value. The values in the headers map would automatically be used with each call to connect() and reconnect().

I also added a method to remove a header since a user may not want to send every header on every call to reconnect().
Btw, I actually have a commit ready for this, but I don't want that commit to go in to the pending PR, since that PR covers something entirely unrelated. I will send a new PR for this issue once the pending one is merged/closed.

[ItsHarper]

@haruntuncay You can submit multiple PRs at once, you just need to submit them from different branches. In general it's a good idea to work on code in feature branches, rather than your master branch like you did with the currently open PR.

[haruntuncay]

@haruntuncay You can submit multiple PRs at once, you just need to submit them from different branches. In general it's a good idea to work on code in feature branches, rather than your master branch like you did with the currently open PR.

Thank you for the notice. I wasn't aware that we could submit multiple PRs simultaneously.

[marci4]

Resolved #868