MAGETWO-57397: Update gallery entry via API doesn't work - for mainline

Author: Mulderua

app/code/Magento/Catalog/Test/Unit/Model/Product/Gallery/GalleryManagementTest.php

@@ -200,6 +200,7 @@ public function testUpdate()
         $this->productMock->expects($this->once())->method('getMediaGalleryEntries')
             ->willReturn([$existingEntryMock]);
         $entryMock->expects($this->once())->method('getId')->willReturn($entryId);
+        $entryMock->expects($this->once())->method('getFile')->willReturn("base64");
         $entryMock->expects($this->once())->method('setId')->with(null);
 
         $this->productMock->expects($this->once())->method('setMediaGalleryEntries')