Bug: can remove slash from expire field and spaces from card number field

Users can go back and remove the slash from the expire field and then cannot add it back unless i remove everything

Try this:
1. Open http://jessepollak.github.io/payment/example/
2. Type 1234 in the expire field
3. Press the left arrow 2 times
4. Press backspace two times.

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Spaces can also be removed in the same way from the card number field.

jquery.payment doesn't have these bugs: http://stripe.github.io/jquery.payment/example/