[5.5] Fix connection resolver and static side effect

[hastinbe]

The following tests fail on a fresh clone of 5.5 when you run phpunit --filter DatabaseEloquentModelTest

1) Illuminate\Tests\Database\DatabaseEloquentModelTest::testCreateMethodSavesNewModel
Error: Call to a member function connection() on null

2) Illuminate\Tests\Database\DatabaseEloquentModelTest::testMakeMethodDoesNotSaveNewModel
Error: Call to a member function connection() on null

3) Illuminate\Tests\Database\DatabaseEloquentModelTest::testForceCreateMethodSavesNewModelWithGuardedAttributes
Error: Call to a member function connection() on null

4) Illuminate\Tests\Database\DatabaseEloquentModelTest::testWithoutMethodRemovesEagerLoadedRelationshipCorrectly
Error: Call to a member function connection() on null

5) Illuminate\Tests\Database\DatabaseEloquentModelTest::testEagerLoadingWithColumns
Error: Call to a member function connection() on null

There's static state from a previous test that is affecting other tests.