-Fixed MetadataTypeAttribute reflection bug by using CustomAttributeData

-Fixed immutable collections bug by getting type from known assembly
-Added support from running tests on aspnetcore50

Src/Newtonsoft.Json.Tests/Serialization/SerializationEventAttributeTests.cs

@@ -37,12 +37,16 @@ using System.Runtime.Serialization;
 using System.Text;
 using Newtonsoft.Json.Serialization;
 using Newtonsoft.Json.Tests.TestObjects;
-#if !NETFX_CORE
-using NUnit.Framework;
-#else
+#if NETFX_CORE
 using Microsoft.VisualStudio.TestPlatform.UnitTestFramework;
 using TestFixture = Microsoft.VisualStudio.TestPlatform.UnitTestFramework.TestClassAttribute;
 using Test = Microsoft.VisualStudio.TestPlatform.UnitTestFramework.TestMethodAttribute;
+#elif ASPNETCORE50
+using Xunit;
+using Test = Xunit.FactAttribute;
+using Assert = Newtonsoft.Json.Tests.XUnitAssert;
+#else
+using NUnit.Framework;
 #endif
 using Newtonsoft.Json.Linq;
 using Newtonsoft.Json.Utilities;
@@ -282,7 +286,7 @@ namespace Newtonsoft.Json.Tests.Serialization
 }", json);
         }
 
-#if !(NETFX_CORE || PORTABLE)
+#if !(NETFX_CORE || PORTABLE || ASPNETCORE50)
         public class SerializationEventContextTestObject
         {
             public string TestMember { get; set; }
@@ -313,7 +317,7 @@ namespace Newtonsoft.Json.Tests.Serialization
         }
 #endif
 
-#if !PORTABLE
+#if !(PORTABLE || ASPNETCORE50)
         public void WhenSerializationErrorDetectedBySerializer_ThenCallbackIsCalled()
         {
             // Verify contract is properly finding our callback