Proxy [[Enumerate]]: exhaust iterator and throw on non-strings.

See:
tc39#160
tc39#161

spec.html

@@ -8639,7 +8639,15 @@ <h1>[[Enumerate]] ()</h1>
           1. Return ? _target_.[[Enumerate]]().
         1. Let _trapResult_ be ? Call(_trap_, _handler_, &laquo; _target_ &raquo;).
         1. If Type(_trapResult_) is not Object, throw a *TypeError* exception.
-        1. Return _trapResult_.
+        1. Let _array_ be ArrayCreate(0).
+        1. Let _nextIndex_ be 0.
+        1. Repeat
+          1. Let _next_ be ? IteratorStep(_trapResult_).
+          1. If _next_ is *false*, return CreateArrayIterator(_array_, `"value"`).
+          1. Let _nextValue_ be ? IteratorValue(_next_).
+          1. If Type(_nextValue_) is not String, throw a *TypeError* exception.
+          1. Perform ! CreateDataPropertyOrThrow(_array_, ToString(ToUint32(_nextIndex_)), _nextValue_).
+          1. Let _nextIndex_ be _nextIndex_ + 1.
       </emu-alg>
       <emu-note>
         <p>[[Enumerate]] for proxy objects enforces the following invariants:</p>