Fix list SQL Server composite foreign keys

This PR fixes listing composite foreign keys in <code>SQLServerSchemaManager</code>. Currently the schema manager creates a foreign key object for each column in a foreign key constraint returned by the platform's <code>listTableForeignKeysSQL</code>, This is ok for foreign keys consisting of only one column but fails for composite foreign key constraints. The test <code>testListForeignKeysComposite</code> in <code>SQLServerSchemaManagerTest</code> currently fails but is fixed with this PR.