Stop internal exception on close (#2)

Stream can be closed twice causing an exception
Currently code traps this exception but there was no need for it in the fristr place.

Author: AdrianSoundy

source/nanoFramework.System.Net.Http/Http/System.Net._OutputNetworkStreamWrapper.cs

@@ -129,7 +129,7 @@ public override void Close()
                 m_headersSend();
             }
 
-            m_Stream.Close();
+            if (m_Stream != null) m_Stream.Close();
             m_Stream = null;
             m_Socket = null;
         }