use $unset in update queries when the attribute has been unset on the
model
remove $unset for attrs not explicitly set to `undefined`
added test for attrs that were not explicitly unset
remove log statement
clean up new testing db
use `model.unset` to detect when to use `$unset`

than relying on an attribute being explicitly `undefined`
`models.unsetAttrs` instead of `model.unset`
Merge pull request #15 from paulbdavis/unset

use $unset in update queries when the attribute has been unset on the model