Make sure assert_select can assert body tag

This reverts commit f93df52845766216f0fe36a4586f8abad505cac4, reversing
changes made to a455e3f4e9dbfb9630d47878e1239bc424fb7d13.

Conflicts:
	actionpack/lib/action_controller/test_case.rb
	actionview/lib/action_view/test_case.rb