[BUGFIX beta] Add warning to variable {{input type}}, with working tests

This is an update to #5056, which should fix the tests.
@rwjblue Actually, I've found that to not be the case. The impetus for this PR was troubleshooting someone's issue on irc that was this case.

See http://emberjs.jsbin.com/jezicuqe/4/edit for a failing example. The checkbox actually does re-render, but it actually uses `TextField` to do it, which causes `checked` to not work.