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

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.

Read the nrepl-client request path and inspect how requests are prepared before being sent to nREPL. Ensure false and nil keys are omitted before encoding, then add or run tests covering requests containing those values and confirm valid keys remain unchanged.

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