Fix deserialization of ID type

Deserialization of ID types caused an exception if the field was
nullable.

Author: hth313

tests/FSharp.Data.GraphQL.Tests/FSharp.Data.GraphQL.Tests.fsproj

@@ -69,6 +69,7 @@
     <Compile Include="AspNetCore/TestSchema.fs" />
     <Compile Include="AspNetCore/InvalidMessageTests.fs" />
     <Compile Include="AspNetCore/SerializationTests.fs" />
+    <Compile Include="ResponseJsonTests.fs" />
     <Compile Include="Program.fs" />
   </ItemGroup>
 
@@ -81,6 +82,13 @@
     <ProjectReference Include="..\..\src\FSharp.Data.GraphQL.Shared\FSharp.Data.GraphQL.Shared.fsproj" />
     <ProjectReference Include="..\..\src\FSharp.Data.GraphQL.Server\FSharp.Data.GraphQL.Server.fsproj" />
     <ProjectReference Include="..\..\src\FSharp.Data.GraphQL.Server.AspNetCore\FSharp.Data.GraphQL.Server.AspNetCore.fsproj" />
+    <ProjectReference Include="..\..\src\FSharp.Data.GraphQL.Client.DesignTime\FSharp.Data.GraphQL.Client.DesignTime.fsproj" />
     <ProjectReference Include="..\..\src\FSharp.Data.GraphQL.Server.Middleware\FSharp.Data.GraphQL.Server.Middleware.fsproj" />
   </ItemGroup>
-</Project>
+</Project>
+  <ItemGroup Condition="$(OS) == Unix">
+    <Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
+  <Import Project="..\..\netfx.props" />
+  <Import Project="..\..\.paket\Paket.Restore.targets" />
+</Project>

tests/FSharp.Data.GraphQL.Tests/ResponseJsonTests.fs

@@ -0,0 +1,73 @@
+module FSharp.Data.GraphQL.Tests.ResponseJsonTests
+
+open System
+open Xunit
+open Helpers
+open FSharp.Data.GraphQL
+open FSharp.Data.GraphQL.Client
+
+// Local provider should be able to be created from local introspection json file.
+// This schema is adjusted to have some less common type names in order to test them.
+type Provider = GraphQLProvider<"introspection-altered-types.json">
+
+
+[<Fact>]
+let ``Should be able to parse nullable ID``() =
+    let op = Provider.Operation<"""query TestQuery {
+      hero(id:"1000") {
+          id,
+          name,
+          appearsIn,
+          homePlanet,
+          friends {
+            ... on Human {
+              name,
+              id
+            }
+            ... on Droid {
+              name,
+              id
+            }
+          }
+        }
+      }""">
+    let xop = op()
+    let result1 = xop.ParseResult("""{
+       "documentId": 2018203290,
+       "data": {
+         "hero": {
+          "id": "1000",
+          "__typename": "Human",
+          "name": "Luke Skywalker",
+          "appearsIn": [
+            "NewHope",
+            "Empire",
+            "Jedi"
+          ],
+          "homePlanet": "Tatooine",
+          "friends": [
+            {
+              "name": "Han Solo",
+              "id": "1002",
+              "__typename": "Human"
+            },
+            {
+              "name": "Leia Organa",
+              "id": "1003",
+              "__typename": "Human"
+            },
+            {
+              "name": "C-3PO",
+              "id": "2000",
+              "__typename": "Droid"
+            },
+            {
+              "name": "R2-D2",
+              "id": "2001",
+              "__typename": "Droid"
+            }
+          ]
+        }
+       }
+    }""")
+    ()