Add HTTP return code 418 - #3

Thanks, @RinkAttendant6! Released with version 1.1.1.

Note that the spec @RinkAttendant6 commits is actually an aprils fools. Note the date https://tools.ietf.org/html/rfc2518 also, it quotes: "This memo provides information for the Internet community. It does not specify an Internet standard of any kind.". HTTP 418 is according to IANA unassigned: https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml

Regarding #3 (comment)

It might be a joke by the IETF, but there are certainly sites that implement it. Besides, I doubt that a future RFC would use the status code, and if there are, another change to this library would be warranted.

@RinkAttendant6 where is my HTTP BREW then? :(

@martinmine There's four servers that advertise support for the BREW method in their Access-Control-Allow-Method response header: https://www.shodan.io/search?query=BREW+http+access-control-allow-methods

@RinkAttendant6 thnx