Using wp_encode_json

[ideadude]

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• Have you followed the Contributing guidelines?
• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

Fixes issues reported by the .org Plugin Check plugin.

There is still an error RE the contributors section of the readme, but that is a false positive. FWIW that issue is being fixed in the Plugin Check plugin here: WordPress/plugin-check#276

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?