feat: introduce Object.ensurePromise

Allows validation of thenable objects

object/ensure-promise.js

@@ -0,0 +1,9 @@
+'use strict';
+
+var safeToString = require('../safe-to-string')
+  , isPromise    = require('./is-promise');
+
+module.exports = function (value) {
+	if (!isPromise(value)) throw new TypeError(safeToString(value) + " is not a thenable");
+	return value;
+};

object/index.js

@@ -11,6 +11,7 @@ module.exports = {
 	create:                     require('./create'),
 	ensureNaturalNumber:        require('./ensure-natural-number'),
 	ensureNaturalNumberValue:   require('./ensure-natural-number-value'),
+	ensurePromise:              require('./ensure-promise'),
 	eq:                         require('./eq'),
 	every:                      require('./every'),
 	filter:                     require('./filter'),

test/object/ensure-promise.js

@@ -0,0 +1,18 @@
+"use strict";
+
+module.exports = function (t, a) {
+	var promise;
+	a.throws(function () { t(); }, TypeError);
+	a.throws(function () { t(null); }, TypeError);
+	a.throws(function () { t("promise"); }, TypeError);
+	a.throws(function () { t({}); }, TypeError);
+	a.throws(function () { t(function () {}); }, TypeError);
+	a.throws(function () { t({ then: {} }); }, TypeError);
+	promise = { then: function () {} };
+	a(t(promise), promise);
+	promise = function () {};
+	promise.then = {};
+	a.throws(function () { t(promise); }, TypeError);
+	promise.then = function () {};
+	a(t(promise), promise);
+};