Tighten up event init dictionary definition

Closes #11.

Since both places in the spec initialize both attributes immediately, and the dictionary contains required for promise and = null for reason, there is no need to specify in prose a default value for the attributes anymore.

README.md

@@ -108,14 +108,14 @@ interface PromiseRejectionEvent : Event {
 };
 
 dictionary PromiseRejectionEventInit : EventInit {
-  Promise<any>? promise;
-  any reason;
+  required Promise<any>? promise;
+  any reason = null;
 };
 ```
 
-The `promise` attribute must return the value it was initialised to. When the object is created, this attribute must be initialised to null. It represents the promise which this notification is about.
+The `promise` attribute must return the value it was initialised to. It represents the promise which this notification is about.
 
-The `reason` attribute must return the value it was initialized to. When the object is created, this attribute must be initialised to null. It represents the rejection reason for the promise.
+The `reason` attribute must return the value it was initialized to. It represents the rejection reason for the promise.
 
 ## [WindowEventHandlers](https://html.spec.whatwg.org/multipage/webappapis.html#windoweventhandlers)