Add sum() to sugar.d.ts.

The nullary version of sum() was missing in sugar.d.ts (ony the unary
version was present).

sugar/sugar.d.ts

@@ -2820,7 +2820,7 @@ interface Array<T> {
 	* @see subtract
 	**/
 	subtract(args: T[]): T[];
-	
+
 	/**
 	* Sums all values in the array.
 	* @param map Property on each element in the array or callback function to sum up the elements.
@@ -2836,6 +2836,11 @@ interface Array<T> {
 	**/
 	sum(map: string): number;
 
+    /**
+    * @see sum
+    **/
+	sum(): number;
+
 	/**
 	* @see sum
 	**/