Avoid unicode LookupError

`six.text_type` is `unicode` on Python 2 and `str` on Python 3. Both of these
functions accept multiple parameters: the second is the `encoding` parameter.
[According to the documentation](http://docs.python.org/2/library/functions.html#unicode): "The encoding parameter is a string giving the
name of an encoding; if the encoding is not known, `LookupError` is raised."
Because `LookupError` does not inherit from `TypeError`, it is not caught by
the try/catch blocks, and can result in Flask-Restful returning a 400 status
code response.

This commit changes the default type to a wrapper around `six.text_type` that
only takes one parameter. This way, an encoding will never be specified, and it
will never raise a `LookupError`.