Fix contract validation & Playbook.tx_to_json

[bchamagne]

Description

2 bugfixes while working on playground:

1. Contract validation was wrong resulting in the possibility to overwrite a contract
2. Encode16 the content when calling the API

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Tested while working on the playground

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules