Add tests and/or sanitization to payload

[yeraydavidrodriguez]

A payload can be passed to the message, that can be used by the final queue user to store its specific data storage needs.

This is implemented in the constructor of the Message class. However, this is assigned to the class as-is.

We should perform the following tests to ensure that the payload is robust to those cases and, in case that is not, perform sanitization actions before assigning the value to it.

Proposed actions (list not exhaustive) are:

• Payloads with JSON and non-JSON structure
• Payloads with problematic characters (quotes, double-quotes, non-ASCII characters...)
• Payloads with malicious code (code injection prevention)