http_client: fix muti part http2 request not working

When the request body is quite big and it's split but only the first part is send multiple time instead of sending all parts.

src/flb_http_client_http2.c

@@ -366,7 +366,8 @@ static ssize_t http2_data_source_read_callback(nghttp2_session *session,
     }
     else {
         if (content_length > 0) {
-            memcpy(buf, stream->request.body, content_length);
+            memcpy(buf,
+                   &stream->request.body[body_offset], content_length);
 
             stream->request.body_read_offset += content_length;
         }