Extract IncludeDirectives.

[beauby]

Extract IncludeDirectives into the jsonapi_parser gem, in an attempt to share work between ActiveModelSerializers and jsonapi-resources (c.f. rails-api/active_model_serializers#1685).

c.f. https://github.com/beauby/jsonapi_parser/blob/master/lib/json/api/include_directive.rb

[beauby]

Note that Travis is failing because of dependency resolution failing for rails 4.1 (jsonapi_parser currently requires activesupport ~> 4.2, but this is not a strong requirement at all, I'll change it if there is interest in this PR).

[beauby]

A bit of context here:

• The proposed IncludeDirective implementation has an underlying structure of a Hash. example:

{
  posts: {
    author: {},
    comments: {
      author: {}
    }
  }
}

whereas the current implementation has a somewhat similar structure, with the addition of an included flag for each relationship (which in practice is always true), and the sub-relationships are stored under an :include_related key.

• The proposed implementation has the additional benefit of handling wildcards (* for "all relationships", ** for "all relationships and their relationships, and so on recursively"). Those are not part of the spec, and are therefore optional (using the :allow_wildcard option to the IncludeDirective constructor – not enabled in this PR). There currently are discussions whether the spec should allow some kind of wildcard, and whichever choice is made would be easily added in the proposed implementation.

[lgebhardt]

@beauby Good idea. I'm looking at the parsing logic for requests again (in anticipation of support for Operations in the spec), so this is good timing. We should coordinate on the features.

The included flag is a remnant from before the spec was finalized, and can probably be removed (I doubt anyone is using it).

[beauby]

Rebased.