Add more unit tests to rpc/impl/TransactionSign [RIPD-480]

By adding a mock it is possible to test the transactionSign
function without interacting with the ledger.  This is the
smallest change I could come up with that allows transactionSign
to be unit tested.

The unit tests are white boxed.  Each test case is a result
of examining the code and identifying behavior associated with
different JSON fields.  That means the tests are not based on
requirements, they are based on observed behavior.