Fix writing of unicode strings to streams in Python 2

When writing a unicode string to a file in Python 2, the string needs to be encoded before writing. This was handled properly in Logbook 0.4.1, but a bug was introduced in 0.4.2 which prevented this encoding from working properly.

The resulting problem can be reproduced easily by running this simple script with Logbook 0.4.2 on Python 2:

    import logbook

    log_handler = logbook.FileHandler('foo.log')
    log_handler.push_application()
    log = logbook.Logger('Foo')
    str = u'\u0431'
    log.info(str)

It results in this error within format_and_encode:

    UnicodeEncodeError: 'ascii' codec can't encode character u'\u0431' in position 30: ordinal not in range(128)

This commit restores the correct Logbook 0.4.1 behavior for Python 2.