Fix test_gzip failure on OS X. The failure was a result of trying to fflush

a file that wasn't open for writing.  Patch by Antoine Pitrou.

Lib/gzip.py

@@ -362,7 +362,7 @@ class GzipFile(io.BufferedIOBase):
         if self.mode == WRITE:
             # Ensure the compressor's buffer is flushed
             self.fileobj.write(self.compress.flush(zlib_mode))
-        self.fileobj.flush()
+            self.fileobj.flush()
 
     def fileno(self):
         """Invoke the underlying file object's fileno() method.