Fixed bug with json files content being escaped in api

After update to recent grape env['api.format'] does not work any more.
Use content_type for rendering raw json files content

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
 Merge branch 'bug/blob_escaped_json' into 'master'

Fixed bug with json files content being escaped in api

Fixes #1035