Match the URL schemes 

Currently, if the scheme of the URL passed in does not match the scheme of `WP_CONTENT_URL`, then the search and replace `str_replace( WP_CONTENT_URL, WP_CONTENT_DIR, $src )` does not occur, and the `$src` string remains a URL rather than being converted to a file path.

For example, we had this situation, where the replace failed (note different schemes):

```php
str_replace( 'http://example.com/somesite/wp-content', '/srv/www/someproject/htdocs/wp-content', 'https://example.com/somesite/wp-content/themes/some-theme/assets/less/wp-admin.less?ver=3.7.1' )
```

This pull request matches the schemes if they differ, forcing the `$src` scheme to the scheme of `WP_CONTENT_URL`.