Change 32-bit large file fix to use bool instead of long.

(Patch originally by thobrla@, see https://github.com/GoogleCloudPlatform/gsutil/commit/edf1c888e96048d65d8d643e2131cc144b5a7bb6)
diff --git a/apitools/base/py/stream_slice.py b/apitools/base/py/stream_slice.py
index 7c06b4b..9901b81 100644
--- a/apitools/base/py/stream_slice.py
+++ b/apitools/base/py/stream_slice.py
@@ -22,7 +22,7 @@
   def __nonzero__(self):
     # For 32-bit python2.x, len() cannot exceed a 32-bit number; avoid
     # accidental len() calls from httplib in the form of "if this_object:".
-    return self.__max_bytes
+    return bool(self.__max_bytes)
 
   @property
   def length(self):