Add support for promisifying prototype functions (#69)

Fixes #67

index.js

@@ -52,7 +52,9 @@ module.exports = (input, options) => {
 
 	let ret;
 	if (objType === 'function') {
-		ret = (...args) => options.excludeMain ? input(...args) : processFn(input, options)(...args);
+		ret = function (...args) {
+			return options.excludeMain ? input(...args) : processFn(input, options).apply(this, args);
+		};
 	} else {
 		ret = Object.create(Object.getPrototypeOf(input));
 	}

test.js

@@ -42,6 +42,10 @@ function FixtureClass() {
 }
 util.inherits(FixtureClass, FixtureParent);
 FixtureClass.prototype.method1 = fixture;
+FixtureClass.prototype.method2Cb = function (cb) {
+	setImmediate(() => cb(null, this.instanceValue1));
+};
+FixtureClass.prototype.method2Async = m(FixtureClass.prototype.method2Cb);
 FixtureParent.prototype.overriddenValue1 = 4;
 FixtureClass.prototype.value1 = 'neo';
 
@@ -269,3 +273,8 @@ test('class support - options.include over options.exclude', t => {
 	t.is(typeof pInstance.parentMethod1().then, 'function');
 	t.not(typeof pInstance.grandparentMethod1(() => {}).then, 'function');
 });
+
+test('promisify prototype function', async t => {
+	const instance = new FixtureClass();
+	t.is(await instance.method2Async(), 72);
+});