Bugfix Cling._transpose_environ and MediaCling.debug_cling

Bugfixes:

* _transpose_environ treated base_url as a string when it was a tuple. By accident, len('/static/') - 1 == len(base_url) + 1 == 7. Breaks for a url of any other length.
* MediaCling's method declaration of debug_cling was insufficient to override the instance attribute of the same name.  Replaced with explicit declaration.