Make AesEncrypter disposable (#19)

SystemExtensions.NetStandard.Security/Encryption/AesEncrypter.cs

@@ -6,7 +6,7 @@ using System.Security.Utilities;
 
 namespace System.Security.Encryption
 {
-    public sealed class AesEncrypter : ISymmetricEncrypter
+    public sealed class AesEncrypter : ISymmetricEncrypter, IDisposable
     {
         private readonly Aes aes;
 
@@ -218,5 +218,10 @@ namespace System.Security.Encryption
         {
             return new MemoryStream(value);
         }
+
+        public void Dispose()
+        {
+            this.aes.Dispose();
+        }
     }
 }