Use PUT request for storing push notification subscription (#274)

Author: danroth27

docs/09-progressive-web-app.md

@@ -145,7 +145,7 @@ Also add the `SubscribeToNotifications` method to `OrdersClient`.
 ```csharp
 public async Task SubscribeToNotifications(NotificationSubscription subscription)
 {
-    var response = await httpClient.PostAsJsonAsync("notifications/subscribe", subscription);
+    var response = await httpClient.PutAsJsonAsync("notifications/subscribe", subscription);
     response.EnsureSuccessStatusCode();
 }
 ```

save-points/09-progressive-web-app/BlazingPizza.Client/OrdersClient.cs

@@ -34,7 +34,7 @@ public async Task<int> PlaceOrder(Order order)
 
         public async Task SubscribeToNotifications(NotificationSubscription subscription)
         {
-            var response = await httpClient.PostAsJsonAsync("notifications/subscribe", subscription);
+            var response = await httpClient.PutAsJsonAsync("notifications/subscribe", subscription);
             response.EnsureSuccessStatusCode();
         }
     }

src/BlazingPizza.Client/OrdersClient.cs

@@ -34,7 +34,7 @@ public async Task<int> PlaceOrder(Order order)
 
         public async Task SubscribeToNotifications(NotificationSubscription subscription)
         {
-            var response = await httpClient.PostAsJsonAsync("notifications/subscribe", subscription);
+            var response = await httpClient.PutAsJsonAsync("notifications/subscribe", subscription);
             response.EnsureSuccessStatusCode();
         }
     }