Fix "not keeping old RELS" (#107)

There was plenty wrong here when telling IMI to keep existing RELS-EXT relationships in place:
1.- We were forcing a new RELST-EXT datastream to be created in the batch object by adding the CMODEL
2.- We were opening the door to IMI's getDatastreamsforUpdate to take new RELS EXT in consideration. This unsets any existing RESL datastreams and also only adds CMODEL to the other Update modes.

includes/islandora_multi_batch.inc

@@ -1276,25 +1276,25 @@ class IslandoraMultiBatchObject extends IslandoraBatchObject {
     $existing_object = islandora_object_load($this->id);
     $relsext_mode = isset($this->preprocessorParameters['object_maping']['relsext_row']['relsext_mode']) ? $this->preprocessorParameters['object_maping']['relsext_row']['relsext_mode'] : 'NEW';
     if ($existing_object) {
-      // just in case i really want to allow Object transmutation?
-      $this->addContentModelRelationships(); 
+
       // Copy existing member relationships over;
       if ($relsext_mode == 'NEW') {
+          $this->addContentModelRelationships();
       // Sets new member relationships over
         $this->setValidRelationshipsforUpdate($existing_object);
       // sets scratch inheritance (new ones only)
         $this->inheritXacmlPoliciesForUpdate($existing_object); 
       }
       if  ($relsext_mode == 'MERGE') {
-
+        $this->addContentModelRelationships();
         $this->setValidRelationshipsforUpdate($existing_object);
       // sets scratch inheritance (new ones only)
         $this->inheritXacmlPoliciesForUpdate($existing_object); 
         $this->copyFedoraRelsUnique($existing_object);
         $this->copyOtherRelsUnique($existing_object);
       }
       if  ($relsext_mode == 'JOIN') {
-
+        $this->addContentModelRelationships();
         $this->setValidRelationshipsforUpdate($existing_object);
       // sets scratch inheritance (new ones only)
         $this->inheritXacmlPoliciesForUpdate($existing_object); 
@@ -1303,7 +1303,9 @@ class IslandoraMultiBatchObject extends IslandoraBatchObject {
       }
 
       if ($relsext_mode == 'OLD') {
-          // Do nothing.
+        // We don´t want any RELS DS to be processed.
+          unset($this['RELS-EXT']);
+          unset($this['RELS-INT']);
       }
     }
   }