Updating relationship gives sometimes wrong duplicate relationship result

When you create / update a relationship there is a test to see if the relationship already exists.
public static function checkDuplicateRelationship inside CRM_Contact_BAO_Relationship
This returns the wrong result under some conditions.

To reproduce create two relationships between person X and person Y with a diiferent start / end date.
If you now update one of those relations through the api, with only the id specified ( for example civicrm_api3('Relationship', 'create', array('id' => 'id'
, is_active => '0') ) you'll get a duplicate relation error.
This is because inside CRM_Contact_BAO_Relationship::checkDuplicateRelationship the date's only get added to the query, when they're specified in the given params. (see if (array_key_exists($dateField, $params)) line 865). If you omit those, they're not specified in the resulting query which causes the result query to see the other relation as a duplicate.

I can work around this by specifiing the dates expliciltly when updating a contact through the api.
civicrm_api3('Relationship', 'create', array('id' => 'id'
'start_date' => date
'end_date => 'date
, is_active => '0')

I'm not sure how this should be solved best. My guess would be to pre-escape the duplicate test when an id is explicitly given, since then we always update, and don't need to check to avoid a duplicate creation.
I'll try a PR.