Views preview shows the JSON output encoded

Problem/Motivation

In #3549810: Views REST Export: HTML entities "&" in the fields are encoded e.g. as "&" in JSON response, which is inconsistent and unexpected the views preview output of the JSON tricked me - it shows the result encoded, which is != the real views output that doesn't have special characters encoded (which is correct for JSON).

Steps to reproduce

• Configure a REST Export view, for example containing node title and body - containing special characters like &
• Set the row options for these fields to "Unmodified output" so that special characters are not escaped unintentionally
• Look at the views preview and see them escaped, while the real output is unescaped - they may also trick others

Proposed resolution

Ensure the views preview result equals the real result of the view

Bonus: Beautify / format JSON output

Remaining tasks

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