Avoid extra allocation for static HTTP handling

src/http.h

@@ -17,6 +17,7 @@ struct mg_http_message {
   struct mg_http_header headers[MG_MAX_HTTP_HEADERS];  // Headers
   struct mg_str body;                                  // Body
   struct mg_str head;                                  // Request + headers
+  struct mg_str chunk;    // Chunk for chunked encoding,  or partial body
   struct mg_str message;  // Request + headers + body
 };