offloaded: reinstate fix for windows tests

in a discussion the other day, brian had asked me to try removing this fix, since
it leads to double-closing the reader.  since on my windows box, the test failures
I'd experienced were related to the ConnectionLost exception problem, and this
close didn't see to make a difference to test results, I agreed.

turns out that the buildbot's environment does fail without this fix, even with
the exception fix, as I'd kind of expected.

it makes sense, because the reader (specifically the file handle) must be closed
before it can be unlinked. at any rate, I'm reinstating this, in order to fix the
windows build

src/allmydata/offloaded.py

@@ -92,6 +92,7 @@ class CHKUploadHelper(Referenceable, upload.CHKUploader):
     def _finished(self, res):
         (uri_extension_hash, needed_shares, total_shares, size) = res
         upload_results = {'uri_extension_hash': uri_extension_hash}
+        self._reader.close()
         os.unlink(self._encoding_file)
         self._finished_observers.fire(upload_results)
         self._helper.upload_finished(self._storage_index)