change id injection, and account for more permutations

in particular, editing a record to add habtm associations won't
necessarily have a model->alias top key
add another test ensuring for an empty submission

in this case - the id should be injected only. This would be rare and
probably invalid in terms of application logic - who knows if someone
want's to "bump" a record's last-updated timestamp or something though
move injectPrimaryKey to a seperate method
Merge pull request #85 from jippi/feature/edit-better-id-injection

change id injection, and account for more permutations