path.write fails bad with unicode

this a followup to pytest-dev/pytest#605

```python
# -*- 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
