NEP-5 approve fix

[brianlenz]

• Updated the NEP-5 approve function so that it always replaces an existing value. This ensures the balance validation is meaningful, and it allows enforcement that the amount is never negative. In order to clear an allowance now, all you'd need to do is set the allowance to 0, which has the further optimization now of deleting the approval from storage.

proposed fix for #11

[shargon]

I think that should be merged. This have sense

[localhuman]

Thanks @brianlenz!