Made ESP8266WebServer::client() return a reference (fixes #7075)

[bblanchon]

edit from maintainer: fixes #7075

[d-a-v]

There should be no functionality changes.
ClientContext is the same in both case.
ClientType may be ssl, pointers to structs are shared too in both case.

edit: a quick test shows things are still running fine

[devyte]

I am in favor of this, in fact I completely agree to move away from return-by-value. However, there is worry that this could cause pain due to moving away from the Arduino way of return-by-value (object slicing or whatever), as has happened in the past.
Whatever the case, this is a breaking change due to a signature change, so at best it must be targeted for v3.