fix #4029: Use enum item.Name instead of Enum.GetName to get enum value

This is the propsed fix from the original repo of the code: neuecc/Utf8Json#129

Author: sklirg

src/Elasticsearch.Net/Utf8Json/Formatters/EnumFormatter.cs

@@ -166,7 +166,7 @@ static EnumFormatter()
 			foreach (var item in type.GetFields().Where(fi => fi.FieldType == type))
 			{
 				var value = item.GetValue(null);
-				var name = Enum.GetName(type, value);
+				var name = item.Name;
 				var dataMember = item.GetCustomAttributes(typeof(DataMemberAttribute), true)
 					.OfType<DataMemberAttribute>()
 					.FirstOrDefault();