fix(fcm): Setting ApplicationName property for the batch client (#268)

Author: hiranya911

FirebaseAdmin/FirebaseAdmin.Tests/Messaging/FirebaseMessagingClientTest.cs

@@ -194,6 +194,8 @@ public async Task SendAllAsync()
             Assert.Equal("projects/fir-adminintegrationtests/messages/5903525881088369386", response.Responses[1].MessageId);
             Assert.Equal(1, handler.Calls);
 
+            var userAgent = handler.LastRequestHeaders.UserAgent.First();
+            Assert.Equal("fire-admin-dotnet", userAgent.Product.Name);
             Assert.Equal(2, this.CountLinesWithPrefix(handler.LastRequestBody, VersionHeader));
             Assert.Equal(2, this.CountLinesWithPrefix(handler.LastRequestBody, ApiFormatHeader));
         }

FirebaseAdmin/FirebaseAdmin/Messaging/FirebaseMessagingClient.cs

@@ -68,6 +68,7 @@ internal FirebaseMessagingClient(Args args)
             {
                 HttpClientFactory = args.ClientFactory,
                 HttpClientInitializer = args.Credential,
+                ApplicationName = ClientVersion,
             });
             this.sendUrl = string.Format(FcmSendUrl, args.ProjectId);
             this.restPath = this.sendUrl.Substring(FcmBaseUrl.Length);