Authorization error response is not valid JSON when api_only

[f3ndot]

Steps to reproduce

1. Use Doorkeeper's master (presently v5.0.0.rc1)
2. Set initializer to use api_only
3. Make a GET request to /oauth/authorize?client_id=badclient

Expected behavior

Response should be something like

{
  "error": "The authorization server does not support this response type.",
  // potentially more metadata:
  "error_type": "unsupported_response_type",
  "status": 401
}

Actual behavior

We get just raw text back, despite the response header Content-Type: application/json:

The authorization server does not support this response type.

Here is a well-formed JSON response from a successful request:

The issue appears to be

doorkeeper/app/controllers/doorkeeper/authorizations_controller.rb

Lines 34 to 41 in bf36149

	def render_error
	if Doorkeeper.configuration.api_only
	render json: pre_auth.error_response.body[:error_description],
	status: :bad_request
	else
	render :error
	end
	end

where there is no root key or JSON object established. Just a string.

Proposed solution

Prefix the error with a error key. Not sure if there is already an existing pattern elsewhere. If so, we should adopt that pattern.