[btcsuite/btcrpcclient] ee3a86: Make listsinceblock block hash optional.

[Josh Rickmar]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcrpcclient
  Commit: ee3a86c44b4cd49992f2b5c830fcced60c7bcc0f
      
https://github.com/btcsuite/btcrpcclient/commit/ee3a86c44b4cd49992f2b5c830fcced60c7bcc0f
  Author: Josh Rickmar <jr...@conformal.com>
  Date:   2015-06-01 (Mon, 01 Jun 2015)

  Changed paths:
    M wallet.go

  Log Message:
  -----------
  Make listsinceblock block hash optional.

Before, calling ListSinceBlock(Async) would create an empty string
from the nil block hash.