Update spec to mention rejecting with a new RangeError (#179)

spec.bs

@@ -1523,13 +1523,10 @@ For now, see [https://github.com/wicg/observable#operators](https://github.com/w
        :: [=Reject=] |p| with the passed in <var ignore>error</var>.
 
        : [=internal observer/complete steps=]
-       :: [=Resolve=] |p| with {{undefined}}.
+       :: [=Reject=] |p| with a new {{RangeError}}.
 
           Note: This is only reached when the source {{Observable}} completes *before* it emits a
-          single value; in this case, resolving with {{undefined}} is harmless but makes it
-          difficult to distinguish between the first value trule being {{undefined}} and premature
-          completion. See <a href=https://github.com/WICG/observable/issues/132>#132</a> for
-          discussion on this.
+          single value.
 
     1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to [=this=] given |internal
        observer| and |internal options|.