Added toJSONObject util; (#5247)

Fixed AxiosError.toJSON method to avoid circular references;

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/core/AxiosError.js

@@ -45,7 +45,7 @@ utils.inherits(AxiosError, Error, {
       columnNumber: this.columnNumber,
       stack: this.stack,
       // Axios
-      config: this.config,
+      config: utils.toJSONObject(this.config),
       code: this.code,
       status: this.response && this.response.status ? this.response.status : null
     };