Fix PushSubscription.toJSON was the wrong return type (scala-js#346)

Update PushSubscription to better reflect w3 standards:
-Change the return type of method toJSON
-Add missing properties (expirationTime, options)

Author: FabioPinheiro

src/main/scala/org/scalajs/dom/experimental/push/PushManager.scala

@@ -14,7 +14,7 @@ import scala.scalajs.js.typedarray.{ArrayBuffer, Uint8Array}
  *
  * MDN
  *
- * The Push API is currently specified here: [[http://www.w3.org/TR/2015/WD-push-api-20151215/]]
+ * The Push API is currently specified here: [[https://www.w3.org/TR/2018/WD-push-api-20181026/]]
  */
 @js.native
 trait PushManager extends js.Object {
@@ -79,6 +79,23 @@ trait PushSubscription extends js.Object {
    */
   val endpoint: String = js.native
 
+  /**
+   * The expirationTime read-only property of the PushSubscription interface returns a
+   * DOMHighResTimeStamp of the subscription expiration time associated with the push
+   * subscription, if there is one, or null otherwise.
+   *
+   * MDN
+   */
+  val expirationTime: Double = js.native
+
+  /**
+   * The options read-only property of the PushSubscription interface is an object containing
+   * containing the options used to create the subscription.
+   *
+   * MDN
+   */
+  val options: PushSubscriptionOptions = js.native
+
   /**
    * The getKey method retrieves keying material that can be used for encrypting and authenticating messages.
    */
@@ -98,7 +115,7 @@ trait PushSubscription extends js.Object {
    *
    * MDN
    */
-  def toJSON(): String = js.native
+  def toJSON(): js.Dictionary[js.Any] = js.native
 }
 
 /**