editorial: call UnwrapNumberFormat in NumberFormat.prototype.formatTo…

…Parts

Call UnwrapNumberFormat in Intl.NumberFormat.prototype.formatToParts in
order to make it more consistent to format and resolvedOptions.

Fixes: tc39#274

spec/numberformat.html

@@ -625,7 +625,7 @@ <h1>Intl.NumberFormat.prototype.formatToParts ( _value_ )</h1>
       <emu-alg>
         1. Let _nf_ be the *this* value.
         1. If Type(_nf_) is not Object, throw a *TypeError* exception.
-        1. If _nf_ does not have an [[InitializedNumberFormat]] internal slot, throw a *TypeError* exception.
+        1. Let _nf_ be ? UnwrapNumberFormat(_nf_).
         1. Let _x_ be ? ToNumber(_value_).
         1. Return ? FormatNumberToParts(_nf_, _x_).
       </emu-alg>