Convert unicode tokens to 'str'

This was resulting in unicode cookie values (and thus headers) on
Python 2 causing mod_wsgi to complain:
        TypeError: expected string object for header value

PEP 3333 also says:
  "Native" strings (which are always implemented using the type named
  str) that are used for request/response headers and metadata

So mod_wsgi is right to complain about unicode headers and Pyramid is
wrong to send them.