fix for CRM-12006 : plugins_url() takes care of https page load as well

In wordpress ,on HTTPS pages (like contribution page), the civicrm js and css script files includes load with url starting from 'http:://' which produce 'insecure content' warnings

The problem is that WP_PLUGIN_URL will always return the siteurl (as specified in Settings > General) and thus does not adjust from http:... to https:... when needed. 

plugins_url() takes care of above