Demonstrate issue 17

I ran into the same issue as #17 , so I created a small test case for it. The problematic css was this in my case:
```
      @media screen and (max-width: 400px) {
        @-ms-viewport {
          width: 320px;
        }
      }
```
This is from Bootstrap 3.0.0. The latest Bootstrap (3.2.0) seems to be ok with mincss.