fully match yield* by doing one last IteratorValue (#19)

* fully match yield* by doing one last IteratorValue (15.5.5 step 7.a.v.1)

* simplify

Author: michaelficarra

spec.emu

@@ -27,6 +27,7 @@ copyright: false
         1. Repeat, while _innerAlive_ is *true*,
           1. Let _iteratorResult_ be ? IteratorStep(_iteratorRecord_).
           1. If _iteratorResult_ is ~done~, then
+            1. Perform ? IteratorValue(_iteratorResult_).
             1. Set _innerAlive_ to *false*.
           1. Else,
             1. Let _completion_ be Completion(GeneratorYield(_iteratorResult_)).