Warning message for non-plugin class

[mcspronko]

In case child class extends Magento\Framework\App\Config\Value class with afterSave() method a Warning message is shown:
Plugin afterSave function should have at least two parameters.

Note: there is no plugin declaration in di.xml file.

[zlik]

@mcspronko, we'll have someone look into this. Thank you for reporting.

[zhooravlik]

Fixed by commit 561266f.
PR#63 created, will be merged to master soon.
Closing the issue.