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.

In this case & and = are encoded as well because treated as a part of value