pallet-contract: Exempted contract doesn't upgrade their last rent payed

[gui1117]

Currently if a contract is exempted from rent (because it holds enough funds for instance) then the last deduct_block (block where it payed rent for the last time) is not updated: https://github.com/paritytech/substrate/blob/master/frame/contracts/src/rent.rs#L181

Thus if a contract has enough funds to pay for 10MB and then it reaches 11MB or just lost some funds, the rent will be from the last time the contract had payed rent because deduct block would be the last time it has been charged.