BugFix: Reduce an empty observable

This fixes issue https://github.com/Netflix/RxJava/issues/423

The fix is based on this comment by @headinthebox: https://github.com/Netflix/RxJava/issues/423#issuecomment-27642532
Merge pull request #474 from benjchristensen/issue-423-reduce-empty-observable

BugFix: Reduce an empty observable