Added 'Content-Type': 'text/html, utf-8' on response header defenition

Hi guys,

Added charset definition  for header's ```Content-Type```

As you know, specifying a character set early for your HTML documents allows the browser to begin executing scripts immediately.

Form here https://developers.google.com/speed/docs/best-practices/rendering#SpecifyCharsetEarly :

> Microsoft notes: "we continue to strongly recommend that web developers specify the CHARSET in the HTTP Content-Type response header, as this ensures that the performance benefit of the Lookahead Downloader is realized".