Fix issue with postgresql urlWithoutDb
Fix issue with postgresql urlWithoutDb

See: https://github.com/gobuffalo/buffalo/issues/836
If the db is not precised, postgresql takes the username as the database to connect on.
To avoid a connection problem if the user db is not here, we use the default "postgres"
db, just like the other client tools do.

u007/pop,markbates/pop,u007/pop,markbates/pop