Add Support for Multiple Values in Filter

[drush]

JSONAPI implementations may support multiple values for a filter to reduce the number of queries on the server side: http://jsonapi.org/recommendations/#filtering

GET /comments?filter[post_id]=1,2 HTTP/1.1

The resulting query should be an IN query in rails:

Comment.where(post_id: values).to_sql 
=> SELECT * FROM comments where post_id IN(val1, va2)

[drush]

SUPPORTED: GET /comments?filter[attr][]=1&filter[attr][]=2 HTTP/1.1

NOT SUPPORTED: GET /comments?filter[attr][0]=1&filter[attr][1]=2 HTTP/1.1 GET /comments?filter[post_id]=1,2 HTTP/1.1

[nicklandgrebe]

Gonna leave this open because though I like the solution, I want to continually improve JSON API adherence