Исправления для корректной работы методов, связанных с треками (#20)

* New enum value

* YTrackAlbumPair ToString for user uploaded tracks

src/Yandex.Music.Api/Models/Common/YTrackSource.cs

@@ -1,10 +1,15 @@
+using System.ComponentModel;
 using System.Runtime.Serialization;
 
 namespace Yandex.Music.Api.Models.Common
 {
     public enum YTrackSource
     {
         [EnumMember(Value = "OWN")]
-        Own
+        Own,
+
+        [EnumMember(Value = "UGC")]
+        [Description("User Generated Content")]
+        UGC
     }
 }

src/Yandex.Music.Api/Models/Track/YTrackAlbumPair.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Text;
 
 namespace Yandex.Music.Api.Models.Track
 {
@@ -9,7 +10,13 @@ public class YTrackAlbumPair : IEquatable<YTrackAlbumPair>
 
         public override string ToString()
         {
-            return $"{Id}:{AlbumId}";
+            var sb = new StringBuilder(Id);
+            if (string.IsNullOrWhiteSpace(AlbumId))
+                return sb.ToString();
+
+            sb.Append(':');
+            sb.Append(AlbumId);
+            return sb.ToString();
         }
 
         #region IEquatable