fixed cookie encoding

prompted by BFW:
https://travis-ci.org/FND/tiddlywebplugins.bfw/builds/13645282

there `environ['HTTP_COOKIE']` is a Unicode string (Python 2), but
(oddly) `Cookie:BaseCookie`'s `load` method checks for `type("")` rather
than `basestring`

presumably this is a bug in the latest version of wsgi-intercept:
`environ` data should still be UTF-8-encoded, no?