Avoid allocating empty List<string>s in DomainList.ctor()

Routes are rarely used these days, so wait until the user starts adding
domains to the list before allocating a growable list.

MimeKit/DomainList.cs

@@ -42,7 +42,7 @@ namespace MimeKit {
 	public class DomainList : IList<string>
 	{
 		readonly static byte[] DomainSentinels = new [] { (byte) ',', (byte) ':' };
-		readonly List<string> domains;
+		IList<string> domains;
 
 		/// <summary>
 		/// Initialize a new instance of the <see cref="DomainList"/> class.
@@ -70,7 +70,13 @@ public DomainList (IEnumerable<string> domains)
 		/// </remarks>
 		public DomainList ()
 		{
-			domains = new List<string> ();
+			domains = Array.Empty<string> ();
+		}
+
+		void EnsureAllocated ()
+		{
+			if (domains == Array.Empty<string> ())
+				domains = new List<string> ();
 		}
 
 		#region IList implementation
@@ -113,6 +119,7 @@ public void Insert (int index, string domain)
 			if (domain is null)
 				throw new ArgumentNullException (nameof (domain));
 
+			EnsureAllocated ();
 			domains.Insert (index, domain);
 			OnChanged ();
 		}
@@ -180,6 +187,7 @@ public void Add (string domain)
 			if (domain is null)
 				throw new ArgumentNullException (nameof (domain));
 
+			EnsureAllocated ();
 			domains.Add (domain);
 			OnChanged ();
 		}