Add HTTP_HEAD to HTTPMethod and parse it (#6413)

* Add HTTP_HEAD to HTTPMethod

* Parse the HTTP_HEAD variant of HTTPMethod from a method string

* Add HTTP_HEAD to the ESP8266WebServer constants

* Skip sending the content of the response if the HTTP method is HEAD method

* Convert the HTTP status code 418  to string
This status code is an easter egg from the IETF and is described in 
[RFC2324](https://tools.ietf.org/html/rfc2324#section-2.3.2)

Author: ZakCodes

libraries/ESP8266WebServer/keywords.txt

@@ -42,6 +42,7 @@ hostHeader	KEYWORD2
 #######################################
 
 HTTP_GET	LITERAL1
+HTTP_HEAD	LITERAL1
 HTTP_POST	LITERAL1
 HTTP_ANY	LITERAL1
 CONTENT_LENGTH_UNKNOWN	LITERAL1

libraries/ESP8266WebServer/src/ESP8266WebServer-impl.h

@@ -480,6 +480,7 @@ void ESP8266WebServerTemplate<ServerType>::send(int code, const String& content_
 
 template <typename ServerType>
 void ESP8266WebServerTemplate<ServerType>::sendContent(const String& content) {
+  if (_currentMethod == HTTP_HEAD) return;
   const char * footer = "\r\n";
   size_t len = content.length();
   if(_chunked) {
@@ -728,6 +729,7 @@ const String ESP8266WebServerTemplate<ServerType>::responseCodeToString(const in
     case 415: return F("Unsupported Media Type");
     case 416: return F("Requested range not satisfiable");
     case 417: return F("Expectation Failed");
+    case 418: return F("I'm a teapot");
     case 500: return F("Internal Server Error");
     case 501: return F("Not Implemented");
     case 502: return F("Bad Gateway");

libraries/ESP8266WebServer/src/ESP8266WebServer.h

@@ -30,7 +30,7 @@
 #include <FS.h>
 #include "detail/mimetable.h"
 
-enum HTTPMethod { HTTP_ANY, HTTP_GET, HTTP_POST, HTTP_PUT, HTTP_PATCH, HTTP_DELETE, HTTP_OPTIONS };
+enum HTTPMethod { HTTP_ANY, HTTP_GET, HTTP_HEAD, HTTP_POST, HTTP_PUT, HTTP_PATCH, HTTP_DELETE, HTTP_OPTIONS };
 enum HTTPUploadStatus { UPLOAD_FILE_START, UPLOAD_FILE_WRITE, UPLOAD_FILE_END,
                         UPLOAD_FILE_ABORTED };
 enum HTTPClientStatus { HC_NONE, HC_WAIT_READ, HC_WAIT_CLOSE };
@@ -194,10 +194,10 @@ class ESP8266WebServerTemplate
   std::unique_ptr<HTTPUpload> _currentUpload;
   int              _postArgsLen;
   RequestArgument* _postArgs;
-    
+
   int              _headerKeysCount;
   RequestArgument* _currentHeaders;
- 
+
   size_t           _contentLength;
   String           _responseHeaders;
 

libraries/ESP8266WebServer/src/Parsing-impl.h

@@ -99,7 +99,9 @@ bool ESP8266WebServerTemplate<ServerType>::_parseRequest(ClientType& client) {
   _chunked = false;
 
   HTTPMethod method = HTTP_GET;
-  if (methodStr == F("POST")) {
+  if (methodStr == F("HEAD")) {
+    method = HTTP_HEAD;
+  } else if (methodStr == F("POST")) {
     method = HTTP_POST;
   } else if (methodStr == F("DELETE")) {
     method = HTTP_DELETE;