added check for string as mapping key for local var.
added check for string as mapping key for local var.

shahankhatch/solidity,ruchevits/solidity,ruchevits/solidity,douglas-larocca/solidity,chriseth/solidity,ruchevits/solidity,debris/solidity,chriseth/solidity-doc-test,ethers/solidity,ruchevits/solidity