IE8 won't iterate properties named "constructor" directly, so move test that property outside of the for-in loop.
Also add test to applyMixin, since enyo.clone won't preserve "constructor" on IE8

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combee@lge.com)
Merge pull request #423 from enyojs/ie8-mixin-fix

IE8 won't iterate properties named "constructor" directly, so move test ...