Fix for onSuccessCallback not called in pipeline

Fixes an issue where the onSuccessCallback method was not being called
when queueing a GetItemScoreInSortedSet command in a pipeline.  The
expected behavior is that the onSuccessCallback method is called after a
pipeline is flushed.
add failing test that's fixed in pull request #127

This failing test is in reference to
https://github.com/ServiceStack/ServiceStack.Redis/pull/127
Merge pull request #127 from alexvodovoz/master

Fixes an issue where the onSuccessCallback method was not being called when queueing a GetItemScoreInSortedSet command in a pipeline.