a little tidying
Remove unneeded decoding code.

django-boundaryservice decoded all feature fields in a try/except block, if an
encoding was set. In 04eeb300, all `str` feature fields were decoded, to "ascii"
by default. In 016dac85, `str` was changed to `bytes`. django-boundaryservice's
code may have been necessary in Django 1.4, but since Django 1.5, Django decodes
feature fields on its own using `force_text`. Note that the change to `bytes`
was an error and should have been changed to `string_types`.