Fix mistake in sync wrapper.

Syncwrapper around service methods returns res after StopIteration
exception. Returning value is evaluated, as soon as first chunk comes,
but in write() method of storage returns choke only and wrapper raises
StopIteration. So I return variable before its assigment.
Fix it with default res value - ""
Merge pull request #31 from noxiouz/master

Fix mistake with write()