And only if the method is not available, attempt to fallback to
the stdout itself if it is a string.

Before 30617c1e, the following kind of check was done:

   >>> type(bytes('foo')) in string_types
   False

And after it, the check was changed to:

   >>> isinstance(bytes('foo'), string_types)
   True

That made the bytes decoding not to be performed at times.  While the
new check is correct, we should favor the attempt to decode, and
only if not available, fallback to the original stdout if it looks
like a string.
Signed-off-by: NCleber Rosa <crosa@redhat.com>