Deadweight#strip strips whitespace

Selectors like `.foo ::-webkit-input-placeholder` were causing Nokogiri to throw an exception. Specifically, `Deadweight#strip` would remove the `::-webkit-input-placeholder`, leaving a trailing whitespace, which Nokogiri doesn't like.

The error message looks something like `Nokogiri::CSS::SyntaxError: unexpected '$' after 'DESCENDANT_SELECTOR'`.