Allow `recordIsLoaded` to be called with a string for the type.

`recordIsLoaded` did not support execution with a string as the `type`
argument, only a DS.Model class reference. This commit makes either
form possible. Consistent with `find`, the `type` argument is
passed through `modelFor` to resolve a class reference.
Merge pull request #1536 from lukemelia/record-is-loaded-with-string

Allow `recordIsLoaded` to be called with a string for the type.