sqlsrv: allow setting primary keys on insert 

I played a bit with sql server recently. After some tests with the schema builder, i found that sql server does not allow to set values to primary key fields, like
`$mapper->id = 5;` (useful for migration scripts or when using combined / composite pkeys)
Instead it'll shout out:
```
Cannot insert explicit value for identity column in table 'TableName'
when IDENTITY_INSERT is set to OFF.
```
And since it seems that there is no way of turning this on globally, or in an extra db->exec before saving the mapper, i added this to the insert query itself.