[wasm] Make WasmHttpMessageHandler the default handler for WebAssembly (#11631)

* Clean up leaks in the WasmHttpMessageHandler.

- Most of the leaks present themselves during a Cancel of a streaming request.

* Make WasmHttpMessageHandler the default handler for WebAssembly

-  Required libraries WebAssembly.Net.Http.dll and WebAssembly.Bindings.dll.

* Clean up spacing

2 files changed, 33 insertions, 7 deletions

diff --git a/mcs/class/System.Net.Http/System.Net.Http/HttpClient.wasm.cs b/mcs/class/System.Net.Http/System.Net.Http/HttpClient.wasm.cs
index 304d9de30c3..86a6edd4027 100644
--- a/mcs/class/System.Net.Http/System.Net.Http/HttpClient.wasm.cs
+++ b/mcs/class/System.Net.Http/System.Net.Http/HttpClient.wasm.cs
@@ -18,13 +18,34 @@ namespace System.Net.Http
         {
 
             if (GetHttpMessageHandler == null)
-                return GetFallback("No custom HttpClientHandler registered");
-
-            var handler = GetHttpMessageHandler();
-            if (handler == null)
-                return GetFallback($"Custom HttpMessageHandler is not valid");
-
-            return handler;
+            {
+                Type type = Type.GetType("WebAssembly.Net.Http.HttpClient.WasmHttpMessageHandler, WebAssembly.Net.Http");
+                if (type == null)
+                    return GetFallback ("Invalid WebAssembly Module? Cannot find WebAssembly.Net.Http.HttpClient.WasmHttpMessageHandler");
+
+                MethodInfo method = type.GetMethod("GetHttpMessageHandler", BindingFlags.Static | BindingFlags.NonPublic);
+                if (method == null)
+                    return GetFallback ("Your WebAssembly version does not support obtaining of the custom HttpClientHandler");
+
+                object ret = method.Invoke(null, null);
+                if (ret == null)
+                    return GetFallback ("WebAssembly returned no custom HttpClientHandler");
+
+                var handler = ret as HttpMessageHandler;
+                if (handler == null)
+                    return GetFallback ($"{ret?.GetType()} is not a valid HttpMessageHandler");
+                    
+                return handler;
+
+            }
+            else
+            {
+                var handler = GetHttpMessageHandler();
+                if (handler == null)
+                    return GetFallback($"Custom HttpMessageHandler is not valid");
+
+                return handler;
+            }                
         }
 
         static HttpMessageHandler GetFallback(string message)
diff --git a/sdks/wasm/WasmHttpMessageHandler.cs b/sdks/wasm/WasmHttpMessageHandler.cs
index 89e0bff577f..dacbeeb18f0 100644
--- a/sdks/wasm/WasmHttpMessageHandler.cs
+++ b/sdks/wasm/WasmHttpMessageHandler.cs
@@ -50,6 +50,11 @@ namespace WebAssembly.Net.Http.HttpClient
             handlerInit();
         }
 
+        private static WasmHttpMessageHandler GetHttpMessageHandler()
+        {
+            return new WasmHttpMessageHandler();
+        }
+
         private void handlerInit()
         {
             window = (JSObject)WebAssembly.Runtime.GetGlobalObject("window");