Fixing an issue when grouping requests could result in long URL

I've got some requests from the Ember app failing with 404.14 and 404.15 errors when app model requested a lot of records. So I found that  RESTAdapter#groupRecordsForFindMany doesn't uri-encode [ ] when groups ids and result of RESTAdapter#pathForType and therefore produce URLs much longer than expected.

So I'm fixing this issue and exposing maxUrlLength as the adapter property so it could be adjusted when needed.

Thats cause by a different bug. If you change line https://github.com/grishace/data/blob/uri_encoded_brackets/packages/ember-data/lib/adapters/rest_adapter.js#L588
to be type = record.constructor, that should fix it for now i believe