Fix pointer arithmetic (#6830)

Actually advance position while looping

Author: devyte

libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp

@@ -678,18 +678,18 @@ int HTTPClient::sendRequest(const char * type, const uint8_t * payload, size_t s
 
         // send Payload if needed
         if (payload && size > 0) {
-            size_t byteswritten = 0;
+            size_t bytesWritten = 0;
             const uint8_t *p = payload;
-            while (byteswritten < size) {
+            while (bytesWritten < size) {
                 int written;
                 int towrite = std::min((int)size, (int)HTTP_TCP_BUFFER_SIZE);
-                written = _client->write(p, towrite);
+                written = _client->write(p + bytesWritten, towrite);
                 if (written < 0) {
                      return returnError(HTTPC_ERROR_SEND_PAYLOAD_FAILED);
                 } else if (written == 0) {
                      return returnError(HTTPC_ERROR_CONNECTION_LOST);
                 }
-                byteswritten += written;
+                bytesWritten += written;
                 size -= written;
             }
         }