Plug.Parsers.JSON should always wrap parsed content

[amplexdenmark]

Currently as documented non object JSON is wrapped in a Map with a "_json" key to the parsed content.
But object JSON is returned as is.

This leads to the situation that parsing:
{"_json":[]}
cannot be distinguished from parsing:
[]

They both leads to the same result: %{"_json" => []}

The fix is not to handle objects specially, but always return the parsed content in a Map with "_json" key.

[josevalim]

Doing so would be a breaking page pretty much to all existing JSON apps out there but we will gladly accept a PR that makes it opt-in.

[amplexdenmark]

Ok, I will consider that.
I'm not that familiar with the inner workings of Plug and how config is passed around, so it will probably be a while before the PR :)

[josevalim]

I believe the options given to Plug.Parsers is available to all parsers, so it could be something like nest_json_at: .... :)