@secured_task decorator bug

[JankesJanco]

When generating security data (for task decorated with @secured_task decorator) using function wrap_plain_data_as_secure from security_helper.py, following error occurs:

File "/usr/local/lib/python3.6/site-packages/zsl/utils/security_helper.py", line 54, in compute_token
sha1hash.update(random_token + secure_token)
TypeError: Unicode-objects must be encoded before hashing

The error occurs only when using Python3. For Python2 it works.