Replace Reflect.has() with the in operator (#304)

index.js

@@ -77,7 +77,7 @@ const deepMerge = (...sources) => {
 			returnValue = [...returnValue, ...source];
 		} else if (isObject(source)) {
 			for (let [key, value] of Object.entries(source)) {
-				if (isObject(value) && Reflect.has(returnValue, key)) {
+				if (isObject(value) && (key in returnValue)) {
 					value = deepMerge(returnValue[key], value);
 				}