Fix GetOrderBy and add tests

tests/Jellyfin.Api.Tests/Helpers/RequestHelpersTests.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Generic;
+using Jellyfin.Api.Helpers;
+using Jellyfin.Data.Enums;
+using Xunit;
+
+namespace Jellyfin.Api.Tests.Helpers
+{
+    public class RequestHelpersTests
+    {
+        [Theory]
+        [MemberData(nameof(GetOrderBy_Success_TestData))]
+        public void GetOrderBy_Success(IReadOnlyList<string> sortBy, IReadOnlyList<SortOrder> requestedSortOrder, (string, SortOrder)[] expected)
+        {
+            Assert.Equal(expected, RequestHelpers.GetOrderBy(sortBy, requestedSortOrder));
+        }
+
+        public static IEnumerable<object[]> GetOrderBy_Success_TestData()
+        {
+            yield return new object[]
+            {
+                Array.Empty<string>(),
+                Array.Empty<SortOrder>(),
+                Array.Empty<(string, SortOrder)>()
+            };
+            yield return new object[]
+            {
+                new string[]
+                {
+                    "IsFavoriteOrLiked",
+                    "Random"
+                },
+                Array.Empty<SortOrder>(),
+                new (string, SortOrder)[]
+                {
+                    ("IsFavoriteOrLiked", SortOrder.Ascending),
+                    ("Random", SortOrder.Ascending),
+                }
+            };
+            yield return new object[]
+            {
+                new string[]
+                {
+                    "SortName",
+                    "ProductionYear"
+                },
+                new SortOrder[]
+                {
+                    SortOrder.Descending
+                },
+                new (string, SortOrder)[]
+                {
+                    ("SortName", SortOrder.Descending),
+                    ("ProductionYear", SortOrder.Ascending),
+                }
+            };
+        }
+    }
+}