Fix !!! 5 trigger error when format = html5

According to ruby haml, even the format set to html5 when !!! 5 still
give you <!DOCTYPE html>

https://github.com/haml/haml/blob/09d14bcbef256ed99ef3bc24bd400db339d19175/test/engine_test.rb#L1043
Merge pull request #49 from linc01n/fix_html5_doctype

Fix !!! 5 trigger error when format = html5