[FIX] change handling of requestLedger options

ledger_index and ledger_hash were being set to true
instead of the specified value, and according to API
docs 'closed','validated', and 'current' can be
supplied to the ledger_index field to get the most
recent of the specified type
Merge pull request #176 from shekenahglory/develop

[FIX] change handling of requestLedger options