Fix a regexp to guess HTML
Merge pull request #932 from ykzts/fix/regexp-to-guess-html

Fix a regexp to guess HTML
Use https URLs wherever available.
Add an entry for #932.
Do not search past 512 characters of a document for an HTML indicator.

This is to avoid wasting time dealing with a huge (non-HTML) XML
document.
Trivial code reduction.
Update libxslt to 2.8.0, libiconv to 1.14, and zlib to 1.2.8.

All tests pass with these latest versions, but there needs some work
before we can update libxml2 to 2.9.x.