Enable Content-Length header in stdio reader

This change turns on the "Content-Length" header expectation for the
MessageReader that is used in the language service host.  The VS Code
plugin now writes out this header for all of its requests.

Author: daviwil

src/PowerShellEditorServices.Transport.Stdio/StdioHost.cs

@@ -142,7 +142,7 @@ async Task ListenForMessages()
             MessageReader messageReader = 
                 new MessageReader(
                     System.Console.In, 
-                    MessageFormat.WithoutContentLength,
+                    MessageFormat.WithContentLength,
                     messageTypeResolver);
 
             MessageWriter messageWriter = 

test/PowerShellEditorServices.Test.Host/LanguageServiceManager.cs

@@ -73,7 +73,7 @@ public void Start()
             this.MessageWriter = 
                 new MessageWriter(
                     this.languageServiceProcess.StandardInput,
-                    MessageFormat.WithoutContentLength,
+                    MessageFormat.WithContentLength,
                     messageTypeResolver);
 
             // Wait for the 'started' event