Limit the encoding detection to 3 bytes for now. This is intended to avoid issues when there's a parser-blocking script in the first 1024 bytes. See also #198. <!-- Reviewable:start --> [<img src="https://reviewable.io/review_button.svg" height="40" alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/html5ever/199) <!-- Reviewable:end -->