Signrawtransaction shouldn't require redeemScript for non-p2sh txins.

The redeemScript functionality broke plain offline signing, this
change makes it only look for that parameter when signing a p2sh
input.
Merge pull request #2264 from gmaxwell/signrawtransaction_for_regular_missing_txins

Signrawtransaction shouldn't require redeemScript for non-p2sh txins.