Allow JSON::Builder#field to accept non-scalar values.

[straight-shoota]

This simplifies writing complex values in json fields:
Instead of json.field "field" { value.to_json(json) } you can simply
write json.field "field", value.

[Sija]

indentation fail

[RX14]

I've wanted this change in 2 projects now, would be great to get another opinion on this so it could be merged. Here's a snippet from a real project:

json.field "avatar" do
  json.object do
    json.field "id", @avatar_id
    json.field "url", "#{config.api_base_url}/v0/avatars/#{@avatar_id || "default"}"
  end
end

this would be written more neatly as

json.field "avatar", {
  id: @avatar_id,
  url: "#{config.api_base_url}/v0/avatars/#{@avatar_id || "default"}"
}