patch - really deprecat assert_tag

I know its really close to 1.2, but this is a really small patch that
really deprecates assert_tag and assert_no_tag. There are articles
and docs that talk about this, but it was never implemented, so the
current code could be confusing or misleading. Here's a patch that
deprecates it, refers users to assert_select instead, and tests it:

http://dev.rubyonrails.org/ticket/7163

thanks,
Rob