Enhancement/enforce jsonapi compliant response format

[EmCousin]

This Pull Request introduces a couple of changes in the way the response is rendered in the following cases :

• When the rendered object is not serializable or is an empty collection, it is not longer returned directly, but nested inside a JSONAPI compliant object

Before :

response = []
render response, meta: { page: 1 }
# => "[]"

After :

response = []
render response, meta: { page: 1 }
# => {\"data\":[],\"meta\":{\"page\":1}}"

Fixes #31