test(sparse-fieldsets): validate the use of the Select extension

test/JsonApiDotNetCoreExampleTests/Acceptance/Spec/SparseFieldSetTests.cs

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+using System.Threading.Tasks;
+using DotNetCoreDocs;
+using DotNetCoreDocs.Writers;
+using JsonApiDotNetCoreExample;
+using Xunit;
+using Microsoft.EntityFrameworkCore;
+using JsonApiDotNetCoreExample.Data;
+using JsonApiDotNetCore.Extensions;
+using JsonApiDotNetCoreExample.Models;
+using System.Linq;
+
+namespace JsonApiDotNetCoreExampleTests.Acceptance.Spec
+{
+    [Collection("WebHostCollection")]
+    public class SparseFieldSetTests
+    {
+        private DocsFixture<Startup, JsonDocWriter> _fixture;
+        private readonly AppDbContext _dbContext;
+
+        public SparseFieldSetTests(DocsFixture<Startup, JsonDocWriter> fixture)
+        {
+            _fixture = fixture;
+            _dbContext = fixture.GetService<AppDbContext>();
+        }
+
+        [Fact]
+        public async Task Can_Select_Sparse_Fieldsets()
+        {
+            // arrange
+            var fields = new string[] { "Id", "Description" };
+            var todoItem = new TodoItem {
+                Description = "description",
+                Ordinal = 1
+            };
+            _dbContext.TodoItems.Add(todoItem);
+            await _dbContext.SaveChangesAsync();
+
+            // act
+            var result = await _dbContext
+                .TodoItems
+                .Where(t=>t.Id == todoItem.Id)
+                .Select(fields)
+                .FirstAsync();
+
+            // assert
+            Assert.Equal(0, result.Ordinal);
+            Assert.Equal(todoItem.Description, result.Description);
+        }
+    }
+}