Handle unicode output during validation

Hi!

This tiny fix allows to validate non-ASCII output. The problem is in interpolation of `str` with `unicode`.
It can also be `from __future__ import unicode_literals` but I'm not sure about side effects.

A snippet to reproduce the error:

    from marshmallow.fields import String

    def validator(output):
        return True

    f = String(validate=validator)
    f.deserialize(u'привет')