Monkeypatch sql_db.dsn like v7

https://github.com/akretion/openerp-server/commit/9f51c13f5088d72a7e83de53172e6336da2918c2
Add test for monkeypatch
Fix compare to test_dsn first time
Merge pull request #16 from gisce/monkeypatch_db_dsn_v6

Monkeypatch db dsn v6