Properly handle media query closing braces

[Umofomia]

I noticed media query blocks weren't properly being handled when they occur entirely before the split. For instance, for the following CSS:

@media print {
  .media-rule { color: black; }
}
.a0 { color: black; }
.a1 { color: black; }
/* ... */
.a4093 { color: black; }
.first-after-split { color: black; }

We would expect that the split would just contain this:

.first-after-split { color: black; }

However, without this fix, it ends up containing this:

@media print {
  .a4093 { color: black; }
  .first-after-split { color: black; }
}

This was caused by two issues:

• extract_part only checked for the closing brace under the if selector_range.cover? selectors_count condition. Because of this, it never knew that the media block had closed. This fix moves that check outside of that condition.
• count_selectors_of_rule incorrectly counted the closing brace of the media query as a selector. This fix takes this into account.

[jhilden]

Thanks a lot.

@matthewd and @esbeka contributed the original implementation of this in #26 maybe they can review your changes to verify that everything is cool? After that I would cut a new release.