Move _STREAM_BOUNDARY before _STREAM_PART (#3720)

The boundary delimiter (_STREAM_BOUNDARY) needs to be send before the
body part (_STREAM_PART) too follow RFC2046. This caused ffplay/ffmpeg
to fail to open the MJPEG stream.

Author: abergmann

Diff for: libraries/ESP32/examples/Camera/CameraWebServer/app_httpd.cpp

@@ -403,16 +403,16 @@ static esp_err_t stream_handler(httpd_req_t *req){
                 }
             }
         }
+        if(res == ESP_OK){
+            res = httpd_resp_send_chunk(req, _STREAM_BOUNDARY, strlen(_STREAM_BOUNDARY));
+        }
         if(res == ESP_OK){
             size_t hlen = snprintf((char *)part_buf, 64, _STREAM_PART, _jpg_buf_len);
             res = httpd_resp_send_chunk(req, (const char *)part_buf, hlen);
         }
         if(res == ESP_OK){
             res = httpd_resp_send_chunk(req, (const char *)_jpg_buf, _jpg_buf_len);
         }
-        if(res == ESP_OK){
-            res = httpd_resp_send_chunk(req, _STREAM_BOUNDARY, strlen(_STREAM_BOUNDARY));
-        }
         if(fb){
             esp_camera_fb_return(fb);
             fb = NULL;