Fix #140: Don't expect 'wsgi.input' to have 'seek' method

The WSGI specification does not require 'wsgi.input' to have a 'seek'
method. However, the gunzipping code in the standard library of Python
2.x uses that method. We work around this by buffering data from a
'wsgi.input' that lacks this method into a temporary file.

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>