Never knew you could contract it has into it's as well.
I remember asking my high school English teacher if I could use the contraction "shouldn't've" for "should not have." She said no. I still disagree.
In vernacular speech, "have" is just contracted left and right; I can actually remember when I realized "'ve" was different from "of", and it wasn't "should of, could of, would of" but "should've, could've, would've".
"That better've not been my cupcake you ate."
"Do you think you should maybe've put your name on it?"