Allow JSONAPI unsuffixed associations keys

As shown here :
https://github.com/json-api/json-api/blob/gh-pages/format/index.md#resource-relationships-
Unsuffixed association keys tests
Unsuffixed association keys doc
Cleaner unsuffixed association keys tests
Merge pull request #639 from gauthier-delacroix/Unsuffixed-default-associations-keys

Allow JSONAPI unsuffixed associations keys