Added HEAD method

[hawwwran]

Added method HEAD, which allows to get just headers in the response from the server. So the server now accepts HEAD method and strips content ig the request method was HEAD.

I added error code 418 I'm a teapot, which is in RFC and there is no need to ignore it.

[codecov-io]

Codecov Report

Merging #3007 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3007   +/-   ##
=======================================
  Coverage   27.82%   27.82%           
=======================================
  Files          20       20           
  Lines        3626     3626           
  Branches      656      656           
=======================================
  Hits         1009     1009           
  Misses       2441     2441           
  Partials      176      176

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[earlephilhower]

Thanks for your PR, but the core and libraries have changed enough that this PR now has a merge conflict.

Could you merge it manually with the latest core, so we can consider it for future releases?

[earlephilhower]

On inspection, we already have a HEAD method in ESP8266WebServer.

Arduino/libraries/ESP8266WebServer/src/ESP8266WebServer.h

Line 33 in 01e9d94

enum HTTPMethod { HTTP_ANY, HTTP_GET, HTTP_HEAD, HTTP_POST, HTTP_PUT, HTTP_PATCH, HTTP_DELETE, HTTP_OPTIONS };

Arduino/libraries/ESP8266WebServer/src/ESP8266WebServer-impl.h

Line 483 in 01e9d94

if (_currentMethod == HTTP_HEAD) return;

Closing.