fix peek() without cb not returning the object

lib/usim.js

@@ -36,7 +36,7 @@
     };
 
     uSim.prototype.peek = function (cb) {
-        this.peekUntil(Infinity, cb);
+        return this.peekUntil(Infinity, cb);
     };
 
     uSim.prototype.streamUntil = function (limit, cb) {