BUG Correct relation saving in DataObject->duplicateRelations()

This caused problems when duplicate() was used in the CMS UI
to duplicate a SiteTree object. Since every object of this type
has a ParentID relation, it copied this empty relation into
new "ghost page".

See https://github.com/silverstripe/silverstripe-cms/issues/689

model/DataObject.php

@@ -499,7 +499,7 @@ class DataObject extends ViewableData implements DataObjectInterface, i18nEntity
 					}
 				}
 			} else {    //one-to-one relation
-				$destinationObject->$name = $relations;
+				$destinationObject->{"{$name}ID"} = $relations->ID;
 			}
 		}
 	}

tests/model/DataObjectDuplicationTest.php

@@ -1,6 +1,8 @@
 <?php
 
 class DataObjectDuplicationTest extends SapphireTest {
+
+	protected $usesDatabase = true;
 	
 	protected $extraDataObjects = array(
 		'DataObjectDuplicateTestClass1',
@@ -28,6 +30,29 @@ class DataObjectDuplicationTest extends SapphireTest {
 		);
 	}
 
+	public function testDuplicateHasOne() {
+		$relationObj = new DataObjectDuplicateTestClass1();
+		$relationObj->text = 'class1';
+		$relationObj->write();
+
+		$orig = new DataObjectDuplicateTestClass2();
+		$orig->text = 'class2';
+		$orig->oneID = $relationObj->ID;
+		$orig->write();
+
+		$duplicate = $orig->duplicate();
+		$this->assertEquals($relationObj->ID, $duplicate->oneID,
+			'Copies has_one relationship'
+		);
+		$this->assertEquals(2, DataObjectDuplicateTestClass2::get()->Count(),
+			'Only creates a single duplicate'
+		);
+		$this->assertEquals(1, DataObjectDuplicateTestClass1::get()->Count(),
+			'Does not create duplicate of has_one relationship'
+		);
+	}
+
+
 	public function testDuplicateManyManyClasses() {
 		//create new test classes below
 		$one = new DataObjectDuplicateTestClass1();