caucase.test: Fix testWSGI on python3.

wsgi.input is specified to be a bytes object, not a string object.
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.Eggs.UnitTest-Master.Python2

00:16:40

passed SlapOS.Eggs.UnitTest-Master.Python3

00:10:36

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:37