New membership doesn't apply sensible defaults for start/end dates

When a new membership is created by CRM_Member_Form_Membership, if the join date is specified, but not the start and end dates, then the membership fails with "There is no valid Membership Status available for selected membership dates." since the omitted start and end dates don't match any status. However if the start and end dates are provided, they are checked against the rules for the specified membership type. It would be more sensible to just default the start and end dates based on the rules for the specified membership type. Implementing this enhancement would be straightforward.