Fix get_metadata to handle unicode strings

get_metadata was failing with non-ascii `author` fields on Python 2. This
brings Python 3's unicode behaviour: all strings are unicode without having to
prefix them with `u''`s.

This works with every python version >= 2.6.

To reproduce the issue, run with the current version of fpm (and python 2.X as the default python):

`fpm -s python -t deb django-push`

The `get_metadata` task fails with a `UnicodeDecodeError`. My patch fixes it.