Mass assignment should be re-enabled after record creation with Eloquent driver.

We ran into a very strange issue in our phpunit tests, where a test would pass if run alone, but would fail when we ran our entire suite.  Eventually we traced it back to the Eloquent::unguard call in the EloquentDatabaseProvide.  Took forever because the TestDummy was used in an entirely different test that ran much earlier than the failing test.

This PR re-enables mass assignment protection after the record is created, which will preserve a more consistent testing environment.