Ensure stream iterator interfaces are related.

Also, add an assertion that the WSGIResponse gets its 'Content-Length'
header correctly set.

src/ZPublisher/Iterators.py

@@ -13,7 +13,7 @@ class IUnboundStreamIterator(Interface):
         """
 
 
-class IStreamIterator(Interface):
+class IStreamIterator(IUnboundStreamIterator):
     """
     An iterator with known length that can be published.
 

src/ZPublisher/tests/test_WSGIPublisher.py

@@ -184,6 +184,8 @@ class WSGIResponseTests(unittest.TestCase):
         response.finalize()
         self.assertTrue(body is response.body)
         self.assertEqual(response._streaming, 0)
+        self.assertEqual(response.getHeader('Content-Length'),
+                         '%d' % len(test_streamiterator.data))
 
     #def test___str__already_wrote_not_chunking(self):
     #    response = self._makeOne()