Add user agent when pushing to inboxes

closes #1

src/FakeRelay.Core/Helpers/MastodonHelper.cs

@@ -19,6 +19,7 @@ public static class MastodonHelper
     public static async Task<string> SendMessageToInboxAsync(string targetHost, string content)
     {
         var client = new HttpClient();
+        client.DefaultRequestHeaders.Add("User-Agent", $"FakeRelay (hosted at {Config.Instance.Host})");
 
         var date = DateTime.UtcNow;