Remove the RAW stream approach

Tests/MQTTnet.TestApp.NetCore/MQTTnet.TestApp.NetCore.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <DebugType>Full</DebugType>
-    <TargetFrameworks>netcoreapp2.0;net451</TargetFrameworks>
+    <TargetFrameworks>netcoreapp2.0;net452</TargetFrameworks>
   </PropertyGroup>
 
   <ItemGroup>

Tests/MQTTnet.TestApp.NetCore/Program.cs

@@ -83,6 +83,12 @@ namespace MQTTnet.TestApp.NetCore
 
         public Task SaveRetainedMessagesAsync(IList<MqttApplicationMessage> messages)
         {
+            var directory = Path.GetDirectoryName(Filename);
+            if (!Directory.Exists(directory))
+            {
+                Directory.CreateDirectory(directory);
+            }
+
             File.WriteAllText(Filename, JsonConvert.SerializeObject(messages));
             return Task.FromResult(0);
         }