added _.isPlainObject definition

lodash/lodash-tests.ts

@@ -638,6 +638,15 @@ result = <boolean>_.isObject({});
 result = <boolean>_.isObject([1, 2, 3]);
 result = <boolean>_.isObject(1);
 
+function Stooge(name, age) {
+  this.name = name;
+  this.age = age;
+}
+
+result = <boolean>_.isPlainObject(new Stooge('moe', 40));
+result = <boolean>_.isPlainObject([1, 2, 3]);
+result = <boolean>_.isPlainObject({ 'name': 'moe', 'age': 40 });
+
 var mergeNames = {
   'stooges': [
     { 'name': 'moe' },

lodash/lodash.d.ts

@@ -2467,6 +2467,13 @@ declare module _ {
 	**/
 	export function isObject(value: any): boolean;
 
+	/**
+	* Checks if value is an object created by the Object constructor.
+	* @param value The value to check.
+	* @return True if value is a plain object, else false.
+	**/
+	export function isPlainObject(value: any): boolean;
+
 	/**
 	* Recursively merges own enumerable properties of the source object(s), that don't resolve 
 	* to undefined into the destination object. Subsequent sources will overwrite property