ESP8266WebServer: Add variadic template version of collectHeaders()

* More user-friendly, less RODATA usage.

eg. `webServer.collectHeaders(F("Content-Type"), F("Origin"));`

In this example, less about 20 bytes than the traditional way.

libraries/ESP8266WebServer/examples/SimpleAuthentication/SimpleAuthentication.ino

@@ -123,11 +123,8 @@ void setup(void) {
   });
 
   server.onNotFound(handleNotFound);
-  //here the list of headers to be recorded
-  const char * headerkeys[] = {"User-Agent", "Cookie"} ;
-  size_t headerkeyssize = sizeof(headerkeys) / sizeof(char*);
   //ask server to track these headers
-  server.collectHeaders(headerkeys, headerkeyssize);
+  server.collectHeaders("User-Agent", "Cookie");
   server.begin();
   Serial.println("HTTP server started");
 }

libraries/ESP8266WebServer/src/ESP8266WebServer-impl.h

@@ -372,7 +372,7 @@ void ESP8266WebServerTemplate<ServerType>::close() {
   _server.close();
   _currentStatus = HC_NONE;
   if(!_headerKeysCount)
-    collectHeaders(0, 0);
+    collectHeaders();
 }
 
 template <typename ServerType>
@@ -557,7 +557,7 @@ void ESP8266WebServerTemplate<ServerType>::_streamFileCore(const size_t fileSize
 }
 
 template <typename ServerType>
-const String& ESP8266WebServerTemplate<ServerType>::pathArg(unsigned int i) const { 
+const String& ESP8266WebServerTemplate<ServerType>::pathArg(unsigned int i) const {
   if (_currentHandler != nullptr)
     return _currentHandler->pathArg(i);
   return emptyString;
@@ -608,7 +608,6 @@ bool ESP8266WebServerTemplate<ServerType>::hasArg(const String& name) const {
   return false;
 }
 
-
 template <typename ServerType>
 const String& ESP8266WebServerTemplate<ServerType>::header(const String& name) const {
   for (int i = 0; i < _headerKeysCount; ++i) {
@@ -618,21 +617,30 @@ const String& ESP8266WebServerTemplate<ServerType>::header(const String& name) c
   return emptyString;
 }
 
-
 template<typename ServerType>
 void ESP8266WebServerTemplate<ServerType>::collectHeaders(const char* headerKeys[], const size_t headerKeysCount) {
-  _headerKeysCount = headerKeysCount + 2;
-  if (_currentHeaders){
+  if (_currentHeaders)
     delete[] _currentHeaders;
-  }
-  _currentHeaders = new RequestArgument[_headerKeysCount];
+  _currentHeaders = new RequestArgument[_headerKeysCount = headerKeysCount + 2];
   _currentHeaders[0].key = FPSTR(AUTHORIZATION_HEADER);
   _currentHeaders[1].key = FPSTR(ETAG_HEADER);
   for (int i = 2; i < _headerKeysCount; i++){
-      _currentHeaders[i].key = headerKeys[i-2];
+      _currentHeaders[i].key = headerKeys[i - 2];
   }
 }
 
+template <typename ServerType>
+template <typename... Args>
+void ESP8266WebServerTemplate<ServerType>::collectHeaders(const Args&... args) {
+  if (_currentHeaders)
+    delete[] _currentHeaders;
+  _currentHeaders = new RequestArgument[_headerKeysCount = sizeof...(args) + 2] {
+    { .key = FPSTR(AUTHORIZATION_HEADER), .value = emptyString },
+    { .key = FPSTR(ETAG_HEADER), .value = emptyString },
+    { .key = args, .value = emptyString } ...
+  };
+}
+
 template <typename ServerType>
 const String& ESP8266WebServerTemplate<ServerType>::header(int i) const {
   if (i < _headerKeysCount)

libraries/ESP8266WebServer/src/ESP8266WebServer.h

@@ -138,6 +138,8 @@ class ESP8266WebServerTemplate
   int args() const;                        // get arguments count
   bool hasArg(const String& name) const;   // check if argument exists
   void collectHeaders(const char* headerKeys[], const size_t headerKeysCount); // set the request headers to collect
+  template<typename... Args>
+  void collectHeaders(const Args&... args); // set the request headers to collect (variadic template version)
   const String& header(const String& name) const; // get request header value by name
   const String& header(int i) const;       // get request header value by number
   const String& headerName(int i) const;   // get request header name by number