Fix #1497: UpdateDescriptor not properly serializing fields when using expressions

Author: gmarz

Diff for: src/Nest/DSL/UpdateDescriptor.cs

@@ -202,7 +202,7 @@ public UpdateDescriptor<TDocument,TPartialDocument> Fields(params Expression<Fun
 			if (!typedPathLookups.HasAny())
 				return this;
 
-			this.Request.RequestParameters.AddQueryString("fields",typedPathLookups);
+			this.Request.RequestParameters.AddQueryString("fields",typedPathLookups.Select(e => (PropertyPathMarker)e).ToList());
 			return this;
 		}
 

Diff for: src/Tests/Nest.Tests.Unit/Nest.Tests.Unit.csproj

@@ -423,6 +423,7 @@
     <Compile Include="Reproduce\Reproduce1199Tests.cs" />
     <Compile Include="Reproduce\Reproduce1146Tests.cs" />
     <Compile Include="Reproduce\Reproduce1440Tests.cs" />
+    <Compile Include="Reproduce\Reproduce1497Tests.cs" />
     <Compile Include="Reproduce\Reproduce1528Tests.cs" />
     <Compile Include="Reproduce\Reproduce1464Tests.cs" />
     <Compile Include="Reproduce\Reproduce629Tests.cs" />

Diff for: src/Tests/Nest.Tests.Unit/Reproduce/Reproduce1497Tests.cs

@@ -0,0 +1,25 @@
+using NUnit.Framework;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Nest.Tests.MockData.Domain;
+
+namespace Nest.Tests.Unit.Reproduce
+{
+	[TestFixture]
+	public class Reproduce1497Tests : BaseJsonTests
+	{
+		[Test]
+		public void UpdateDescriptorFieldsExpressionSerialization()
+		{
+			var update = this._client.Update<ElasticsearchProject>(u => u
+				.Id(1)
+				.Fields(p => p.Name, p => p.Id)
+			);
+			update.ConnectionStatus.RequestUrl.Should().EndWith("fields=name%2Cid");
+		}
+	}
+}