Squash a Python2 SparkData Bug

Python2 is raising TypeErrors when SparkData objects are initialized via unicode strings.  Update `_json_dict()` to check that the json string is an instance of `basestring` and not the `str` subclass.

Author: cmlccie

ciscosparkapi/sparkdata.py

@@ -56,7 +56,7 @@ def _json_dict(json):
     """
     if isinstance(json, dict):
         return json
-    elif isinstance(json, str):
+    elif isinstance(json, basestring):
         return json_pkg.loads(json, object_hook=OrderedDict)
     else:
         error = "'json' must be a dictionary or valid JSON string; " \