[js] Decont the result of the boolification method like the JVM does

The MoarVM seems to accepts less result values from that method, might
be able to to some minor optimization with that knowledge

src/vm/js/nqp-runtime/sixmodel.js

@@ -126,7 +126,7 @@ class STable {
     this.boolificationSpec = {mode: mode, method: method};
     if (mode == 0) {
       this.ObjConstructor.prototype.$$toBool = function(ctx) {
-        return method.$$call(ctx, {}, this).$$toBool(ctx);
+        return method.$$call(ctx, {}, this).$$decont().$$toBool(ctx);
       };
     } else if (mode == 1) {
       this.ObjConstructor.prototype.$$toBool = function(ctx) {