Disable custom server certificate validation due to xplat issue

https://github.com/dotnet/corefx/issues/9728

Author: niemyjski

src/Exceptionless/Submission/DefaultSubmissionClient.cs

@@ -127,7 +127,7 @@ private HttpClient CreateHttpClient(string userAgent) {
 #else
             var handler = new HttpClientHandler { UseDefaultCredentials = true };
 #if !PORTABLE && !NETSTANDARD1_2
-            handler.ServerCertificateCustomValidationCallback = delegate { return true; };
+            //handler.ServerCertificateCustomValidationCallback = delegate { return true; };
 #endif
 #endif