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.