Fix type on py3

This shouldn't have any practical effect, since *any* False-y value for
chunk will give us the correct behavior, but let's be consistent in the
type used with that name.

Change-Id: I1928883792e7a9945f3731ddeea992d14882d679

swift/common/middleware/proxy_logging.py

@@ -361,7 +361,7 @@ class ProxyLoggingMiddleware(object):
                 while not chunk:
                     chunk = next(iterator)
             except StopIteration:
-                chunk = ''
+                chunk = b''
             for h, v in start_response_args[0][1]:
                 if h.lower() in ('content-length', 'transfer-encoding'):
                     break