Update the HTTP Accept header

[hrj]

Our current Accept header is gzip. Firefox has text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 gzip, deflate en-US,en;q=0.5.

For feature and privacy reasons, it would be good to reach parity with Firefox's header.

• Add the html, xml and xhtml mime types
• add language
• Add deflate. Depends on Support deflate #173