db name from connectionstring is not used

When using a connection string for mongodb where the database is provided AND leaving 
db commented out in config the plugin still uses `myApp` as database name.

__mongodb.ini__
```ini
; ... lost of earlier stuff
string=mongodb://usr:passwor@mongoserver:27017/haraka
;db=myApp

; ... lots of later stuff
```

In this case I would expect `haraka`to be used as db name.