Support passing dict to add JSON field

[GodTamIt]

This allows you to now do the following:

doc.add_json(
    "attributes",
    {
        "order": 1.2,
        "target": "submit-button",
        "cart": {"product_id": 133},
        "description": "das keyboard",
    },
)

It also fixes a panic that occurs when passing valid JSON that is not a map, originating here

[cjrh]

This is great, I think it will be quite useful to be able to work with dicts directly. I'd prefer json and dict to be handled in separate methods though.

What do you think?

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[cjrh]

Thanks!