Improve types of the Change class (#403)

Both `before` and `after` fields are now described as non-optional.

Author: merlinnot

changelog.txt

@@ -0,0 +1 @@
+fixed - improved types of the `Change` class to describe both `before` and `after` fields as non-optional

src/cloud-functions.ts

@@ -90,7 +90,7 @@ export interface EventContext {
  * to the event, "after" represents the state after the event.
  */
 export class Change<T> {
-  constructor(public before?: T, public after?: T) {}
+  constructor(public before: T, public after: T) {}
 }
 
 /** ChangeJson is the JSON format used to construct a Change object. */