Use pre-existing response headers to do websocket handshake

[guillaumelamirand]

Motivation:

Same as #5324 but on master.

Conformance:

You should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines