fixed typo

Because PostgreSQL will return the expression with a lot of
parentheses we cannot TRIM() them easily. It is easier and more
correct to adapt to what PostgreSQL returns. That means the declaration
of partial indexes must be updated as follow:

Before:
``options={"where": "other_id IS NULL"}``

After:
``options={"where": "(other_id IS NULL)"}``

And fore more complex conditions, that would be:
``options={"where": "(((id IS NOT NULL) AND (other_id IS NULL)) AND (name IS NULL))"}``

Simplified doModifyLimitQuery per @deeky666 comments, added test case for orderby with parenthetical expression

Added doModifyLimitQuery for SQLServer2012Platform that uses OFFSET... FETCH instead of the awful SQL Server 2008 window function garbage