Add header access using same method as arguments 2

based on @brianensor PR
+sample and some sanity check

libraries/ESP8266WebServer/src/ESP8266WebServer.cpp

@@ -37,10 +37,15 @@ ESP8266WebServer::ESP8266WebServer(int port)
 , _lastHandler(0)
 , _currentArgCount(0)
 , _currentArgs(0)
+,_headerKeysCount(0)
+,_currentHeaders(0)
 {
 }
 
 ESP8266WebServer::~ESP8266WebServer() {
+   if (_currentHeaders)
+     delete[]_currentHeaders;
+  _headerKeysCount = 0;
   if (!_firstHandler)
     return;
   RequestHandler* handler = _firstHandler;
@@ -283,6 +288,48 @@ bool ESP8266WebServer::hasArg(const char* name) {
   return false;
 }
 
+String ESP8266WebServer::header(const char* name) {
+  for (int i = 0; i < _headerKeysCount; ++i) {
+    if (_currentHeaders[i].key == name)
+      return _currentHeaders[i].value;
+  }
+  return String();
+}
+
+void ESP8266WebServer::collectHeaders(const char* headerKeys[], const size_t headerKeysCount) {
+  _headerKeysCount = headerKeysCount;
+  if (_currentHeaders)
+     delete[]_currentHeaders;
+  _currentHeaders = new RequestArgument[_headerKeysCount];
+  for (int i = 0; i < _headerKeysCount; i++){
+    _currentHeaders[i].key = headerKeys[i];
+  }
+}
+
+String ESP8266WebServer::header(int i) {
+  if (i < _headerKeysCount)
+    return _currentHeaders[i].value;
+  return String();
+}
+
+String ESP8266WebServer::headerName(int i) {
+  if (i < _headerKeysCount)
+    return _currentHeaders[i].key;
+  return String();
+}
+
+int ESP8266WebServer::headers() {
+  return _headerKeysCount;
+}
+
+bool ESP8266WebServer::hasHeader(const char* name) {
+  for (int i = 0; i < _headerKeysCount; ++i) {
+    if ((_currentHeaders[i].key == name) &&  (_currentHeaders[i].value.length() > 0))
+      return true;
+  }
+  return false;
+}
+
 String ESP8266WebServer::hostHeader() {
   return _hostHeader;
 }