BOM-618 Don't decode string objects.

common/lib/xmodule/xmodule/lti_module.py

@@ -657,7 +657,7 @@ oauth_consumer_key="", oauth_signature="frVp4JuvT1mVXlxktiAUjQ7%2F1cw%3D"'}
         # so '='' becomes '%3D'.
         # We send form via browser, so browser will encode it again,
         # So we need to decode signature back:
-        params[u'oauth_signature'] = six.moves.urllib.parse.unquote(params[u'oauth_signature']).decode('utf8')
+        params[u'oauth_signature'] = six.moves.urllib.parse.unquote(params[u'oauth_signature']).encode('utf-8').decode('utf8')
 
         # Add LTI parameters to OAuth parameters for sending in form.
         params.update(body)

common/lib/xmodule/xmodule/tests/__init__.py

@@ -176,7 +176,7 @@ def mock_render_template(*args, **kwargs):
     Allows us to not depend on any actual template rendering mechanism,
     while still returning a unicode object
     """
-    return pprint.pformat((args, kwargs)).decode()
+    return pprint.pformat((args, kwargs)).encode().decode()
 
 
 class ModelsTest(unittest.TestCase):