[nrepl-client] Automatically disjoin false/nil keys from requests

[bbatsov]

As you can't encode false/nil with bencode you're actually supposed not to send such flags, but I guess people do it from time to time by mistake. It'd be nice if we simply disjoined them automatically before sending a request to nREPL.

[mikepjb]

Would this be as simple as stripping them out of the object in the nrepl-bencode function on line 402 of nrepl-client.el?

[bbatsov]

That'd be one option, yes. There's the small implication that this way we'll also be stripping empty lists, as unfortunately they are also represented with nil and Elisp doesn't have a real boolean type.