Simplify mergePromise function (#403)

lib/promise.js

@@ -1,23 +1,21 @@
 'use strict';
-const mergePromiseProperty = (spawned, promise, property) => {
-	// Starting the main `promise` is deferred to avoid consuming streams
-	const value = typeof promise === 'function' ?
-		(...args) => promise()[property](...args) :
-		promise[property].bind(promise);
-
-	Object.defineProperty(spawned, property, {
-		value,
-		writable: true,
-		enumerable: false,
-		configurable: true
-	});
-};
+
+const descriptors = ['then', 'catch', 'finally'].map(property => [
+	property,
+	Reflect.getOwnPropertyDescriptor(Promise.prototype, property)
+]);
 
 // The return value is a mixin of `childProcess` and `Promise`
 const mergePromise = (spawned, promise) => {
-	mergePromiseProperty(spawned, promise, 'then');
-	mergePromiseProperty(spawned, promise, 'catch');
-	mergePromiseProperty(spawned, promise, 'finally');
+	for (const [property, descriptor] of descriptors) {
+		// Starting the main `promise` is deferred to avoid consuming streams
+		const value = typeof promise === 'function' ?
+			(...args) => Reflect.apply(descriptor.value, promise(), args) :
+			descriptor.value.bind(promise);
+
+		Reflect.defineProperty(spawned, property, {...descriptor, value});
+	}
+
 	return spawned;
 };