add test: FixtureAdapter doesn't save hasMany records

This failing test proves a bug exists when a record is created in the
store with a hasMany relationships. The relationships are simply lost upon
`phones.createRecord('name', {person: tom})`, where `tom` is the record
that has `phones`.

closes #1606, closes #1750
Merge pull request #2526 from sly7-7/fixtures-hasMany

add regression test: FixtureAdapter correctly save hasMany records