Fix contract deployments

[danjm]

TrezorConnect.ethereumSignTransaction will fail if the to address is undefined, but the toJSON method of ethereumjs-tx will return undefined when dealing with a transaction intended to deploy a contract. With #108 we introduced a change which, for EIP-1559 transactions, we were setting the to address (and other properties) passed to TrezorConnect.ethereumSignTransaction according to the ethereumjs-tx toJSON. This would result in an error. The fix is to explicitly add a normalized version of the to property to the transaction object passed to ethereumSignTransaction