Initialized client to prevent issue cristipufu#14 reported by jluzard…

…o1971 System.NullReferenceException: 'Object reference not set to an instance of an object.'

src/SwiftClient/SwiftClient.cs

@@ -12,7 +12,7 @@ public partial class Client : ISwiftClient, IDisposable
         public SwiftRetryManager RetryManager;
 
         protected ISwiftLogger _logger;
-        protected HttpClient _client;
+        protected HttpClient _client = new HttpClient();
         protected bool _customClient;
 
         public Client() { }