[esnext] implement StringPad spec changes from March TC39 meeting

src/js/harmony-string-padding.js

Index: src/js/harmony-string-padding.js
diff --git a/src/js/harmony-string-padding.js b/src/js/harmony-string-padding.js
index a6c6c474de5503e269b451b3fe89b1173e973453..dc59823fefbb8908a38b7f3f55401834abc38b54 100644
--- a/src/js/harmony-string-padding.js
+++ b/src/js/harmony-string-padding.js
@@ -30,7 +30,8 @@ function StringPad(thisString, maxLength, fillString) {
   } else {
     fillString = TO_STRING(fillString);
     if (fillString === "") {
-      fillString = " ";
+      // If filler is the empty String, return S.
+      return "";
     }
   }