Updates documentation to include reference for RFC2324 (HTTP Status 418)

[TzviPM]

https://www.ietf.org/rfc/rfc2324.txt clearly identifies status code 418 as a viable HTTP status code.

Also, the underlying statuses library does include it as a viable error response code, on line 42 of codes.json (https://github.com/jshttp/statuses/blob/master/codes.json#L42). Coincidentally, 42 is the secret to life in the hit book series and film The Hitchhiker's Guide to the Galaxy.

[codecov]

Codecov Report

Merging #1028 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1028   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files           5        5           
  Lines         369      369           
=======================================
  Hits          368      368           
  Misses          1        1

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