Charset for html

[troelsarvin]

headers has a text_utf8 constructor for ContentType. I suggest a html_utf8 constructor be added, as well, resulting in

Content-Type: text/html; charset=utf-8

Motivation: If the charset is not set for the text/html content type, then it defaults to ISO-8859-1, but in many situations, the charset is really UTF-8.

[jplatte]

Maybe it would be possible to have ContentType::utf8(self) -> Self and deprecate ContentType::text_utf8() in favor of ContentType::text().utf8()?