Finish minting should me called only once
Finish minting should me called only once

Multiple calls to `finishMinting` will emit multiple `MintFinished` events which may be surprising if one rely on `MintFinished` event.
pash7ka/zeppelin-solidity,OpenZeppelin/zeppelin-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zeppelin-solidity,OpenZeppelin/zep-solidity,OpenZeppelin/zep-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/openzeppelin-contracts,pash7ka/zeppelin-solidity