ESP8266WebServer: reduce amount of string copying

Author: igrr

hardware/esp8266com/esp8266/libraries/ESP8266WebServer/src/ESP8266WebServer.cpp

@@ -115,7 +115,7 @@ void ESP8266WebServer::handleClient()
   _handleRequest();
 }
 
-void ESP8266WebServer::sendHeader(String name, String value, bool first) {
+void ESP8266WebServer::sendHeader(const String& name, const String& value, bool first) {
   String headerLine = name;
   headerLine += ": ";
   headerLine += value;
@@ -129,7 +129,7 @@ void ESP8266WebServer::sendHeader(String name, String value, bool first) {
   }
 }
 
-void ESP8266WebServer::send(int code, const char* content_type, String content) {
+void ESP8266WebServer::send(int code, const char* content_type, const String& content) {
   String response = "HTTP/1.1 ";
   response += String(code);
   response += " ";
@@ -155,15 +155,15 @@ void ESP8266WebServer::send(int code, const char* content_type, String content)
   sendContent(response);
 }
 
-void ESP8266WebServer::send(int code, char* content_type, String content) {
+void ESP8266WebServer::send(int code, char* content_type, const String& content) {
   send(code, (const char*)content_type, content);
 }
 
-void ESP8266WebServer::send(int code, String content_type, String content) {
+void ESP8266WebServer::send(int code, const String& content_type, const String& content) {
   send(code, (const char*)content_type.c_str(), content);
 }
 
-void ESP8266WebServer::sendContent(String content) {
+void ESP8266WebServer::sendContent(const String& content) {
   size_t size_to_send = content.length();
   size_t size_sent = 0;
   while(size_to_send) {

hardware/esp8266com/esp8266/libraries/ESP8266WebServer/src/ESP8266WebServer.h

@@ -77,15 +77,15 @@ class ESP8266WebServer
   // code - HTTP response code, can be 200 or 404
   // content_type - HTTP content type, like "text/plain" or "image/png"
   // content - actual content body
-  void send(int code, const char* content_type = NULL, String content = String(""));
-  void send(int code, char* content_type, String content);
-  void send(int code, String content_type, String content);
+  void send(int code, const char* content_type = NULL, const String& content = String(""));
+  void send(int code, char* content_type, const String& content);
+  void send(int code, const String& content_type, const String& content);
 
   void setContentLength(size_t contentLength) { _contentLength = contentLength; }
-  void sendHeader(String name, String value, bool first = false);
-  void sendContent(String content);
+  void sendHeader(const String& name, const String& value, bool first = false);
+  void sendContent(const String& content);
 
-template<typename T> size_t streamFile(T &file, String contentType){
+template<typename T> size_t streamFile(T &file, const String& contentType){
   setContentLength(file.size());
   if (String(file.name()).endsWith(".gz") && 
       contentType != "application/x-gzip" &&