Fixed closing a connection

The inner connection should be unreferenced, but the property should not be removed from the object
Add test case for #688
fix generating COMMENT ON COLUMN statements for quoted identifiers
Fix test purpose
Use method `setExpectedException` instead annotation
Merge branch 'test/#691-failing-test-for-#685' into hotfix/#685-DBAL-997-undefined-_conn-property

Close #691
Merge branch 'hotfix/#688-hotfix-for-#685' into hotfix/#685-DBAL-997-undefined-_conn-property

Close #688
Merge branch 'hotfix/#685-DBAL-997-undefined-_conn-property'

Close #685
Close #688
Close #691
Merge pull request #689 from deeky666/DBAL-1004

[DBAL-1004] Fix generating COMMENT ON COLUMN statements for quoted identifiers