fix error caused by missing content type in html() method (fix #276, #396) (#421)
fix error caused by missing content type in html() method (fix #276, #396) (#421)

* fix error caused by missing content type #276 & #396

* use utf8 for saving html files to S3, leave base64 for pdf and png

adieuadieu/chromeless,adieuadieu/chromeless