Fix WriteFile with unicode data

Fix WriteFile with unicode data

Unicode is fun, indeed:

>>> len(buffer("abc"))
3
>>> len(buffer(u"abc"))
12

So we can't pass unicode data to buffer(), as the result will be to
write the in-memory (usually UTF-32) representation to disk.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>