def METHOD_NAME(self):
    @Request.application
    def application(request):
        return Response("", headers=[
            ('X-Foo', 'bar')
        ])
    application = fixers.HeaderRewriterFix(application, ('X-Foo',), (('X-Bar', '42'),))
    response = Response.from_app(application, create_environ())
    assert response.headers['Content-Type'] == 'text/plain; charset=utf-8'
    assert 'X-Foo' not in response.headers
    assert response.headers['X-Bar'] == '42'