Fix incorrect str/bytes type of session data. (#645)

Works with:
- MySQL column type `TEXT` and `VARBINARY`
- PostgreSQL column type `TEXT` and `bytea`

web/session.py

@@ -239,6 +239,9 @@ def encode(self, session_dict):
 
     def decode(self, session_data):
         """decodes the data to get back the session dict """
+        if isinstance(session_data, str):
+            session_data = session_data.encode()
+
         pickled = decodebytes(session_data)
         return pickle.loads(pickled)
 
@@ -344,7 +347,10 @@ def __getitem__(self, key):
             return self.decode(s.data)
 
     def __setitem__(self, key, value):
-        pickled = self.encode(value)
+        # Remove the leading `b` of bytes object (`b"..."`), otherwise encoded
+        # value is invalid base64 format.
+        pickled = self.encode(value).decode()
+
         now = datetime.datetime.now()
         if key in self:
             self.db.update(