Request renderer: Remove ServerVariable from API in .NET Core

Remove nonsupported property from API

Fixes #40, fixes #39

NLog.Web.AspNetCore/LayoutRenderers/AspNetRequestValueLayoutRenderer.cs

@@ -3,6 +3,7 @@
 #if !NETSTANDARD_1plus
 using NLog.Common;
 using System.Web;
+using System.Web.Routing; 
 #else
 using Microsoft.AspNetCore.Http;
 #endif
@@ -57,12 +58,18 @@ public class AspNetRequestValueLayoutRenderer : AspNetLayoutRendererBase
         /// <docgen category='Rendering Options' order='10' />
         public string Cookie { get; set; }
 
+#if !NETSTANDARD_1plus
+
+        //missing in .NET Core (RC2)
+
         /// <summary>
         /// Gets or sets the ServerVariables item to be rendered.
         /// </summary>
         /// <docgen category='Rendering Options' order='10' />
         public string ServerVariable { get; set; }
 
+#endif
+
         /// <summary>
         /// Gets or sets the Headers item to be rendered.
         /// </summary>
@@ -109,15 +116,14 @@ protected override void DoAppend(StringBuilder builder, LogEventInfo logEvent)
 #endif
 
             }
+#if !NETSTANDARD_1plus
             else if (this.ServerVariable != null)
             {
-#if !NETSTANDARD_1plus
+
                 builder.Append(httpRequest.ServerVariables[this.ServerVariable]);
-#else
 
-                throw new NotSupportedException();
-#endif
             }
+#endif
             else if (this.Header != null)
             {
                 string header = httpRequest.Headers[this.Header];