Cover another corner case.

src/zc/buildout/networkcache.txt

@@ -433,6 +433,8 @@ Check if the parsing url method works correctly, in low-level:
    'id=700c7d5382b01f94e7141'
    >>> get_filename_from_url("http://localhost/tarball-5.1.tar.gz?id=700")
    'tarball-5.1.tar.gz'
+   >>> get_filename_from_url("http://localhost/somedir/?")
+   'somedir'
 
 Check if the networkcache upload method is using the correct method to
 parse the original url and get the file name: