Fixed #10681 -- Work around (by ignoring) invalid ETag headers.

This is a hack to work around problems in the Real World. Apparently, Opera
9.64 has been observed sending malformed headers. We now compromise our high
principles and simply ignore such bad behaviour.

Patch from Ivan Sagalaev.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10370 bcc190cf-cafb-0310-a4f2-bffc1f526a37