Fixes issue with Sql Server driver when loading fixtures

Sqlserver::describe is expecting a model object (Unlike other drivers which work with either a object or string). While this does work under normal conditions, it causes a "Trying to get property of non-object" notice when using fixtures. The same problem also causes the testLimitOffsetHack test to fail.

This change does a simple test to ensure that the $model variable is an object before trying to access a property of it. All SqlServer tests are now passing.
Merge pull request #3441 from dakota/patch-1

Fixes issue with Sql Server driver when loading fixtures