Fix handling of file-already-exists-post-lock #18

Was testing the wrong variable for None-ness. The returned file from
atomic.__enter__() will be None if the file already exists, not the
atomic itself.

csppfetch/__init__.py

@@ -472,7 +472,7 @@ def download_all(url_to_dst, download_stats, do_download, timeout):
         for url,dst in url_to_dst.items():
             atomic = AtomicCreateIfMissing(dst)
             file = atomic.__enter__()
-            if atomic is None:
+            if file is None:
                 atomic.__exit__(None,None,None)
                 download_already_present("download_all post-lock",
                         dst, download_stats, do_download)