support String parameters in Iterator.from and AsyncIterator.from (#250)

Author: michaelficarra

spec.html

@@ -258,6 +258,7 @@ <h1>Iterator.prototype</h1>
         <emu-clause id="sec-iterator.from">
           <h1>Iterator.from ( _O_ )</h1>
           <emu-alg>
+            1. If _O_ is a String, set _O_ to ! ToObject(_O_).
             1. Let _iteratorRecord_ be ? GetIteratorFlattenable(_O_, ~sync~).
             1. Let _hasInstance_ be ? OrdinaryHasInstance(%Iterator%, _iteratorRecord_.[[Iterator]]).
             1. If _hasInstance_ is *true*, then
@@ -336,6 +337,7 @@ <h1>AsyncIterator.prototype</h1>
         <emu-clause id="sec-asynciterator.from">
           <h1>AsyncIterator.from ( _O_ )</h1>
           <emu-alg>
+            1. If _O_ is a String, set _O_ to ! ToObject(_O_).
             1. Let _iteratorRecord_ be ? GetIteratorFlattenable(_O_, ~async~).
             1. Let _hasInstance_ be ? OrdinaryHasInstance(%AsyncIterator%, _iteratorRecord_.[[Iterator]]).
             1. If _hasInstance_ is *true*, then