Set the HTTPS server variables as lower case

System.Web.WorkerRequest checks if the HTTPS server variable equals "on" to determine if a request IsSecure. This check is case sensitive. When this check fails the Request.Url is returned without https.
https://referencesource.microsoft.com/#System.Web/WorkerRequest.cs,620f7ba656959388

Author: ragnarstolsmark

src/Microsoft.AspNetCore.SystemWebAdapters/Adapters/ProxyHeaderModule.Framework.cs

@@ -20,8 +20,8 @@ internal class ProxyHeaderModule : IHttpModule
     private const string ServerPort = "SERVER_PORT";
     private const string ForwardedProto = "x-forwarded-proto";
     private const string ForwardedHost = "x-forwarded-host";
-    private const string On = "ON";
-    private const string Off = "OFF";
+    private const string On = "on";
+    private const string Off = "off";
 
     private readonly IOptions<ProxyOptions> _options;
 

test/Microsoft.AspNetCore.SystemWebAdapters.Framework.Tests/ProxyHeaderModuleTests.cs

@@ -18,8 +18,8 @@ public class ProxyHeaderModuleTests
     private const string ServerName = "SERVER_NAME";
     private const string ServerPort = "SERVER_PORT";
     private const string ServerHttps = "HTTPS";
-    private const string On = "ON";
-    private const string Off = "OFF";
+    private const string On = "on";
+    private const string Off = "off";
 
     [Fact]
     public void NoHeaderChange()