path.write fails bad with unicode

[RonnyPfannschmidt]

this a followup to pytest-dev/pytest#605

# -*- coding: utf-8 -*-

def test_1(tmpdir):
    tmpdir.join('foo').write(u'æ')


def test_2(tmpdir):
    tmpdir.join('foo').write(u'æ'.encode('u8'))


def test_3(tmpdir):
    tmpdir.join('foo').write(u'æ'.encode('l1'))

all fail atm with messy errors, and i believe all should work

[tony]

Happening here https://travis-ci.org/cihai/cihaidata-unihan/jobs/224505963

Works: https://github.com/cihai/cihaidata-unihan/blob/158b099691c053c923006cc4db445902d8367862/tests/test_unihan.py#L209

Breaks: https://github.com/cihai/cihaidata-unihan/blob/fce83fb/tests/test_unihan.py#L173