Try and convert from decimal to the underlying type value, if the type is nullable (#44)

This fixes the underlying problem in #32. Failing test is included.

Author: rmja

SystemTextJsonPatch.Tests/IntegrationTests/PatchDocumentSerializationTest.cs

@@ -1,4 +1,5 @@
 using System.Text.Json;
+using System.Text.Json.Serialization;
 using SystemTextJsonPatch.Exceptions;
 using Xunit;
 
@@ -56,17 +57,21 @@ public void Test()
 		{
 			Class1 obj = new();
 			var sourcePatch = new JsonPatchDocument<Class1>().Replace(c => c.Id, 1000);
-			var deserializedPatchDoc = JsonSerializer.Deserialize<JsonPatchDocument<Class1>>(JsonSerializer.Serialize(sourcePatch));
+			var patchJson = JsonSerializer.Serialize(sourcePatch);
+			var deserializedPatchDoc = JsonSerializer.Deserialize(patchJson, Class1SerializerContext.Default.JsonPatchDocumentClass1);
 
 			sourcePatch.ApplyTo(obj); // success
 			deserializedPatchDoc.ApplyTo(obj); // JsonPatchTestOperationException: The value '1000' is invalid for target location.
 
 			Assert.Equal(1000, obj.Id);
 		}
+	}
 
-		public class Class1
-		{
-			public int? Id { get; set; }
-		}
+	public class Class1
+	{
+		public int? Id { get; set; }
 	}
+
+	[JsonSerializable(typeof(JsonPatchDocument<Class1>))]
+	public sealed partial class Class1SerializerContext : JsonSerializerContext { }
 }

SystemTextJsonPatch/Internal/ConversionResultProvider.cs

@@ -108,6 +108,11 @@ internal static bool TryCopyTo(object? value, Type typeToConvertTo, JsonSerializ
 	// fast-path for converting decimal to target types
 	private static bool TryConvertDecimalToNumber(decimal decimalValue, Type typeToConvertTo, out object? convertedValue)
 	{
+		if (typeToConvertTo.IsGenericType && typeToConvertTo.GetGenericTypeDefinition() == typeof(Nullable<>))
+		{
+			typeToConvertTo = typeToConvertTo.GetGenericArguments()[0];
+		}
+
 		switch (Type.GetTypeCode(typeToConvertTo))
 		{
 			case TypeCode.SByte: