Fix wrong testcase

Quoted from the dump_cookie docs:

>On Python 3 the return value of this function will be a unicode
>string, on Python 2 it will be a native string.

So in both cases it's a native string.

Also related: https://bitbucket.org/pypy/pypy/issue/1849/py2-a-b-csplit-u-returns-wrong-different

Backport from f2b98e127d3edd6e57b43fc901c6628ea6fca8b7

Conflicts:
	werkzeug/testsuite/http.py