utils: fix bug when decoding

utils._decode should return an instance of 'str' instead of 'bytes' in
Python 3.

Fixes: #24

Author: Hunt Xu

etcd3gw/utils.py

@@ -38,7 +38,7 @@ def _decode(data):
     """
     if not isinstance(data, bytes_types):
         data = six.b(str(data))
-    return base64.b64decode(data.decode("utf-8"))
+    return base64.b64decode(data).decode("utf-8")
 
 
 def _increment_last_byte(data):