Better JSON deparsing for curl_translate()

[hadley]

Fixes #258

@mgirlich I'd really appreciate a review of this code once you're back from the break.

[mgirlich]

Awesome! 😄 I didn't yet know about req_body_json_modify(). When I looked it up, I found the documentation a bit confusing:

• there are no examples
• It says

Named components of this data can later be modified with req_body_json_modify().

But here we don't modify the data later but build it from the ground. This makes me wonder if this is really the correct name.

[hadley]

Hmmm, it's actually me who doesn't understand how it works 😬. I'll fix shortly.