flying_scripts_rewrite_html fix

Hello,

thank you for this great plugin!

There are some rare cases when w3tc_process_content is triggered multiple times and then function flying_scripts_rewrite_html will put src to data-src and then it will remove src, so when it will run for the second time, then it will again replace data-src this time with <script> innerHTML that is ofc empty, so it will result in data:text/javascript;base64,

Easy fix is just wrap this condition part if ($script->getAttribute("src")) { ... } else { ... } from flying_scripts_rewrite_html function into another condition checking if there is already data-src, eg. if (!$script->getAttribute("data-src")){ ... }

Can you please implement this fix ASAP?

Thank you very much.