fixes #115: @@toStringTag is writable now

spec.html

@@ -687,7 +687,9 @@ <h1>Iterator.prototype.find ( _fn_ )</h1>
       <emu-clause id="sec-iteratorprototype-@@tostringtag">
         <h1>Iterator.prototype [ @@toStringTag ]</h1>
         <p>The initial value of the @@toStringTag property is the String value "Iterator".</p>
-        <p>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *true* }.</p>
+        <emu-note>
+          <p>Unlike the @@toStringTag on most built-in classes, for web-compatibility reasons this property must be writable.</p>
+        </emu-note>
       </emu-clause>
     </emu-clause>
 
@@ -964,7 +966,9 @@ <h1>AsyncIterator.prototype.find ( _fn_ )</h1>
       <emu-clause id="sec-asynciteratorprototype-@@tostringtag">
         <h1>AsyncIterator.prototype [ @@toStringTag ]</h1>
         <p>The initial value of the @@toStringTag property is the String value "Async Iterator".</p>
-        <p>This property has the attributes { [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *true* }.</p>
+        <emu-note>
+          <p>Unlike the @@toStringTag on most built-in classes, for web-compatibility reasons this property must be writable.</p>
+        </emu-note>
       </emu-clause>
     </emu-clause>
   </emu-clause>