Change predicate json case to snake case

[fracek]

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] I have read the CONTRIBUTING document.
• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[coveralls]

Coverage remained the same at 88.68% when pulling eaa6dfa25abb6d63f3e2078a21dc1f6a1364ce24 on protocol-14-json-fix into 953ec47ef65c2aff784a1bf2ca61c157662d4b6b on master.