Force underscore in pathForType.

AMS Adapter takes for granted that type is always camelized, which is not the case when using ember-jj-abrams-resolver, since it normalizes types with dashes.

Calling  AMS `pathForType` with `pull-request` will return `pull-requests` since `Ember.String.decamelize('pull-request')` returns `pull-request`.

To fix the issue we need to "underscorize" the type explicitly.