Add const char* content to ESP8266WebServer::send() (#6797)

* Add const char* content to ESP8266WebSerer::send()

Supercedes #3492

Allow sending raw binary data or strings directly without conversion to
a String to reduce memory overhead when possible.

From original @timw1971 PR #3492
Added public functions to allow content to be uploaded using const
char*. For some cases, this can remove the need for content to be copied
into a String, and thus can be considerably more space-efficient.

* Fix example formatting

* Make GIF example use static const array

* Make the example really need to use const char*

Make the generated GIF dynamic in the example and move the original to
PROGMEM (since that's where const arrays like this belong).

Author: earlephilhower

libraries/ESP8266WebServer/examples/HelloServer/HelloServer.ino

@@ -67,6 +67,23 @@ void setup(void) {
     server.send(200, "text/plain", "this works as well");
   });
 
+  server.on("/gif", []() {
+    static const uint8_t gif[] PROGMEM = {
+      0x47, 0x49, 0x46, 0x38, 0x37, 0x61, 0x10, 0x00, 0x10, 0x00, 0x80, 0x01,
+      0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0x2c, 0x00, 0x00, 0x00, 0x00,
+      0x10, 0x00, 0x10, 0x00, 0x00, 0x02, 0x19, 0x8c, 0x8f, 0xa9, 0xcb, 0x9d,
+      0x00, 0x5f, 0x74, 0xb4, 0x56, 0xb0, 0xb0, 0xd2, 0xf2, 0x35, 0x1e, 0x4c,
+      0x0c, 0x24, 0x5a, 0xe6, 0x89, 0xa6, 0x4d, 0x01, 0x00, 0x3b
+    };
+    char gif_colored[sizeof(gif)];
+    memcpy_P(gif_colored, gif, sizeof(gif));
+    // Set the background to a random set of colors
+    gif_colored[16] = millis() % 256;
+    gif_colored[17] = millis() % 256;
+    gif_colored[18] = millis() % 256;
+    server.send(200, "image/gif", gif_colored, sizeof(gif_colored));
+  });
+
   server.onNotFound(handleNotFound);
 
   server.begin();

libraries/ESP8266WebServer/src/ESP8266WebServer.h

@@ -127,6 +127,15 @@ class ESP8266WebServerTemplate
   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 send(int code, const char *content_type, const char *content, size_t content_length = 0) {
+    if (content_length == 0) {
+      content_length = strlen_P(content);
+    }
+    send_P(code, content_type, content, content_length);
+  }
+  void send(int code, const char *content_type, const uint8_t *content, size_t content_length) {
+    send_P(code, content_type, (const char *)content, content_length);
+  }
   void send_P(int code, PGM_P content_type, PGM_P content);
   void send_P(int code, PGM_P content_type, PGM_P content, size_t contentLength);