Added differ test checking differ when moving node from document to d…

…ocument fragment.

tests/model/differ.js

@@ -1409,6 +1409,27 @@ describe( 'Differ', () => {
 				] );
 			} );
 		} );
+
+		it( 'should not throw when moving node from document to document fragment', () => {
+			const docFragment = model.change( writer => writer.createDocumentFragment() );
+			const node = model.change( writer => writer.createText( 'foo' ) );
+			const changeSpy = sinon.spy();
+
+			model.change( writer => {
+				writer.insert( node, root );
+			} );
+
+			doc.on( 'change', () => {
+				expect(	() => differ.getChanges() ).to.not.throw();
+				changeSpy();
+			} );
+
+			model.change( writer => {
+				writer.insert( node, docFragment );
+			} );
+
+			sinon.assert.called( changeSpy );
+		} );
 	} );
 
 	function insert( item, position ) {