Ctor single arg: allow plain object, reject null/undefined

Thank you @kub1x

lib/is.js

@@ -16,6 +16,14 @@ const object = function (val) {
   return typeof val === 'object';
 };
 
+/**
+ * Is this value a plain object?
+ * @private
+ */
+const plainObject = function (val) {
+  return object(val) && Object.prototype.toString.call(val) === '[object Object]';
+};
+
 /**
  * Is this value a function?
  * @private
@@ -98,6 +106,7 @@ const invalidParameterError = function (name, expected, actual) {
 module.exports = {
   defined: defined,
   object: object,
+  plainObject: plainObject,
   fn: fn,
   bool: bool,
   buffer: buffer,