Fix Sort method considering different return types

- consider different return types depending on sort options
- adapt SortOptions class to RESP
- introduce a new low level Sort (string, string, params object [])
- rename SendCommandExpectMultiBulkReply to SendExpectDataArray
  because RedisSharp generally uses the term Data for bulk strings
- introduce new method SendExpectStringArray (called by GetKeys)
- extend RedisTest with Sort