Apply a default filter for a relation if none was specified

Follow up to pull #693.

If addFilter() is called for a relation without specifying a default filter (e.g. Groups.Permissions.Code vs. Groups.Permissions.Code:ExactMatch) this patch will apply a default filter.
This default filter is currently hard coded to `search/filter/ExactMatchFilter`, introducing a dependency on that filter.

Willr proposed that the default should be a case insensitive exact match. ExactMatch contains a todo for a case sensitivity switch. So this switch should be set as soon as it is available. Maybe a TODO for that should be added to addFilter.