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
Allowing same Authenticate object to be setup with different settings.
Merge pull request #3445 from renan/2.5-authenticate

2.5 Allowing same Authenticate object to be setup with different settings
Remove wrong return.
test failed. reverted and corrected doc block instead for BC.
Add support for schema alterations to the SqlServer driver