Fix for @extend infinite loop.

In Twitter Bootstrap 3.0.0 converted to SASS ([jlong/sass-bootstrap](https://github.com/jlong/sass-bootstrap/)), the following case in  `_navs.scss` (reduced to minimum failing example) causes an infinite loop to occur at compilation in scssphp:

    .nav-tabs {
      &.nav-justified {
        @extend .nav-justified;
      }
    }
    
    .nav-justified {
      text-align: justify;
    }

The reason is that the `&.nav-justified` that contains the extend is used as its own target.

This pull request adds:
* a test case demonstrating the problem (I wanted to add it to the end of extends.scss, but it caused extra spaces to appear in the output, so I moved it);
* a lucky guess fix that is probably not the right way to correct the problem (I didn’t understand all the underpinnings of the `@extend` processing mechanism) but doesn’t break any tests.