api response content_type should be application/json

Our server requires the http_accept header with our custom api mime type but the response content_type header should be application/json and understandable to any client.
return json response for missing routes
Revert "api response content_type should be application/json"

This reverts commit 8f3a76304bc6efd72effa9ba0f4018453be46819.

JSON API response should reflect the requested HTTP_ACCEPT header.
use normal JSON content types for non-API routes
remove duplicate RecordNotFound rescue
Merge branch 'fix_api_reponse' from pull request #91