* Migrate tests to xunit

* Update sample project to dotnetcore3.1
* Use Sqlite in sample project to run it everywhere
* Fix: Filter with escaped comma
* Fix: Filter "null" does not work with Contains or StartsWith
* Code cleanup: Adjust namespaces, adjust usings

Sieve.Sample/Entities/ApplicationDbContext.cs

@@ -0,0 +1,15 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace Sieve.Sample.Entities
+{
+    public class ApplicationDbContext : DbContext
+    {
+        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options) { }
+
+        public DbSet<Post> Posts { get; set; }
+
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+        }
+    }
+}

Sieve.Sample/Entities/Post.cs

@@ -0,0 +1,31 @@
+using System;
+using System.ComponentModel.DataAnnotations.Schema;
+using Sieve.Attributes;
+
+namespace Sieve.Sample.Entities
+{
+    public class Post
+    {
+        public int Id { get; set; }
+
+        [Sieve(CanFilter = true, CanSort = true)]
+        public string Title { get; set; } = Guid.NewGuid().ToString().Replace("-", string.Empty)[..8];
+
+        [Sieve(CanFilter = true, CanSort = true)]
+        public int LikeCount { get; set; } = new Random().Next(0, 1000);
+
+
+        [Sieve(CanFilter = true, CanSort = true)]
+        public int CommentCount { get; set; } = new Random().Next(0, 1000);
+
+        [Sieve(CanFilter = true, CanSort = true)]
+        public DateTimeOffset DateCreated { get; set; } = DateTimeOffset.UtcNow;
+
+        [Sieve(CanFilter = true, CanSort = true)]
+        [Column(TypeName = "datetime")]
+        public DateTime DateLastViewed { get; set; } = DateTime.UtcNow;
+
+        [Sieve(CanFilter = true, CanSort = true)]
+        public int? CategoryId { get; set; } = new Random().Next(0, 4);
+    }
+}