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 all my observations result stored in baseUrl is '/', and when it finally gets as ajax call url, there's a path for particular type added.