fix #990 nullreference when using the DateMappingDescriptor multi .Fields()

Author: Mpdreamz

Diff for: src/Nest/Domain/Mapping/Types/MultiFieldMapping.cs

@@ -10,13 +10,13 @@ public class MultiFieldMapping : IElasticType
 	{
 		private readonly TypeNameMarker _defaultType;
 
-		public MultiFieldMapping()
-			: this("multi_field")
+		public MultiFieldMapping() 
 		{
 			this.Fields = new Dictionary<PropertyNameMarker, IElasticCoreType>();
+			_defaultType = "multi_field";
 		}
 
-		protected MultiFieldMapping(TypeNameMarker defaultType)
+		protected MultiFieldMapping(TypeNameMarker defaultType) : this()
 		{
 			_defaultType = defaultType;
 		}

Diff for: src/Tests/Nest.Tests.Unit/Reproduce/Reproduce990Tests.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using Elasticsearch.Net;
+using Nest.Tests.MockData.Domain;
+using NUnit.Framework;
+
+namespace Nest.Tests.Unit.Reproduce
+{
+	/// <summary>
+	/// tests to reproduce reported errors
+	/// </summary>
+	[TestFixture]
+	public class Reproduce990Tests : BaseJsonTests
+	{
+		[Test]
+		public void DateMappingShouldNotTrow()
+		{
+			Assert.DoesNotThrow(() =>
+			{
+				var result = _client.Map<ElasticsearchProject>(p => p
+					.DynamicTemplates(d => d
+						.Add(dt => dt
+							.Name("date_template")
+							.Match("*_date")
+							.MatchMappingType("date")
+							.Mapping(mp => mp
+								.Date(date => date
+									.Format("basic_date || date|| yyyy/MM/dd || dd/MM/yyyy || dd-MM-yyyy || MM/dd/yyyy || MM-dd-yyyy")
+									.Fields(f => f
+										.String(fs => fs
+											.Name("raw")
+											.Analyzer("number_delimiter")
+										)
+									)
+								)
+							)
+						)
+					)
+				);
+			});
+		}
+	}
+}