fix StringStream constructor delegation to Stream

pdf.js

@@ -76,7 +76,7 @@ var StringStream = (function () {
         var bytes = new Uint8Array(length);
         for (var n = 0; n < length; ++n)
             bytes[n] = str.charCodeAt(n);
-        this.Stream(bytes);
+        Stream.call(this, bytes);
     }
 
     constructor.prototype = Stream.prototype;