Allow getrawtransaction result to be string.

The getrawtransaction command has recently added a verbose flag which
alters the output.  The previous code made use of the "Hex" field of the
TxRawResult to return the information, but this is not consistent with the
original getrawtransaction RPC call which returns a string when verbose is
false and the TxRawResult JSON object when it is true.

This commit corrects that by allowing the result for getrawtransaction to
be either form.