weboob.tools.application.base: don't do _do_complete_iter for strings

_do_complete_iter should not be called for strings or byte buffers.
boobill's "download" command uses CapDocument.download_document which
returns strings, which should not be split into individual bytes.

The code did work with Python 2 because even though a basestring is
iterable, it doesn't have an __iter__ attribute. In Python 3, it does,
so bytes were iterated as ints.