Removing ruleset nodes from tree may result in two adjacent whitespace
nodes which is not correct AST:
[space, ruleset, space] => [space, space]
To ensure correctness of further processing we should merge such nodes
into one:
[space, space] => [space]
function mergeAdjacentWhitespace(node) {
    var i = node.content.length - 1;
    while (i-- > 0) {
      if (node.get(i).is('space') &&
          node.get(i + 1) && node.get(i + 1).is('space')) {
        node.get(i).content += node.get(i + 1).content;
        node.removeChild(i + 1);
      }
    }
  }