Improve Mono support: Use correct HttpRequestMessage headers field name under Mono (#26)

Author: Misaka-L

StandardSocketsHttpHandler/Net/Http/Extensions/HttpRequestMessageExtensions.cs

@@ -8,9 +8,10 @@ internal static class HttpRequestMessageExtensions
     {
         public static bool HasHeaders(this HttpRequestMessage request)
         {
-            // Note: The field name is _headers in .NET core 
+            // Note: The field name is _headers in .NET core, and headers in .NET Mono / .NET Framework
             bool isDotNetFramework = RuntimeUtils.IsDotNetFramework();
-            string headersFieldName = isDotNetFramework ? "headers" : "_headers";
+            bool isMono = RuntimeUtils.IsMono();
+            string headersFieldName = isDotNetFramework || isMono ? "headers" : "_headers";
             FieldInfo headersField = typeof(HttpRequestMessage).GetField(headersFieldName, BindingFlags.Instance | BindingFlags.NonPublic);
             if (headersField == null && isDotNetFramework)
             {

StandardSocketsHttpHandler/Utils/RuntimeUtils.cs

@@ -18,5 +18,12 @@ public static bool IsDotNetNative()
             string frameworkDescription = RuntimeInformation.FrameworkDescription;
             return frameworkDescription.StartsWith(DotnetFrameworkDescription);
         }
+
+        public static bool IsMono()
+        {
+            const string MonoDescription = "Mono";
+            string frameworkDescription = RuntimeInformation.FrameworkDescription;
+            return frameworkDescription.StartsWith(MonoDescription);
+        }
     }
 }