Update class-disable-author-pages.php

This should work even if wordpress is installed in a subdirectory. With the `$request` method, for `http://example.com/wp/author/mike` the `$request` is `/wp/author/mike` so `stripos` result is not 0.

Here I use `get_query_var()` instead of `$request`.
Merge pull request #1 from smeric/master

Make it work if WordPress installed in a subdirectory