added "onunload" to the list of event handlers to remove

lib/single-file/single-file-core.js

@@ -1843,6 +1843,7 @@ this.singlefile.lib.core = this.singlefile.lib.core || (() => {
 					attributeNames.push(propertyName);
 				}
 			}
+			attributeNames.push("onunload");
 			return attributeNames;
 		}