[1.6.x] Removed an unnecessary anchor in the docs.

Backport of a06808d370 from master
[1.6.x] Dropped fix_IE_for_vary/attach.

This is a security fix. Disclosure following shortly.
[1.6.x] Added additional checks in is_safe_url to account for flexible parsing.

This is a security fix. Disclosure following shortly.
Added release notes for 1.4.13, 1.5.8, 1.6.5.
Updated version numbers for release.
Updated version numbers post-release.