Gadget state does not get modified by `undefined`

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Gadget does not take `undefined` arguments as a change to context.

This makes using of default state almost boiler-plate free. Allows to
easily avoid unnecessary changes by having `undefined` in default state.

For explicite zeroing the state attribute, one should use `null` value.

renderjs.js

@@ -668,7 +668,8 @@
           }
           for (key in state_dict) {
             if (state_dict.hasOwnProperty(key) &&
-                (state_dict[key] !== context.state[key])) {
+                state_dict[key] !== undefined &&
+                state_dict[key] !== context.state[key]) {
               context.state[key] = state_dict[key];
               modification_dict[key] = state_dict[key];
               modified = true;