Bound XML encoding sniffing to the declaration

[domenic]

Stop looking for "encoding" at the first > byte when sniffing an XML declaration in text/html. This prevents text after the declaration from selecting the document's encoding, matching Chromium, Gecko, and WebKit.

Fixes #6939.

• At least two implementers are interested (and none opposed): everyone already implements this
• Tests are written and can be reviewed and commented upon at: existing coverage in xmldecl-2.html, specifically gt-between-xml-and-encoding.htm and gt-between-xml-and-encoding-trail.htm. Both cases and two valid-declaration controls pass in Chromium, Firefox, and WebKit.
• Implementation bugs are filed: not needed
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

[annevk]

Nice catch.

I wondered if encoding="blah > blah" was also problematic, but it seems that in such a case we will always return failure so that is fine.