Implement IssuesClient and interface

This required updating our serialization strategy so we handle enums
better.

Octokit/Models/RepositoryIssueRequest.cs

@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace Octokit
+{
+    public class RepositoryIssueRequest : IssueRequest
+    {
+        /// <summary>
+        /// Identifies a filter for the milestone. Use "*" for issues with any milestone.
+        /// Use the milestone number for a specific milestone. Use the value "none" for issues with any milestones.
+        /// </summary>
+        public string Milestone { get; set; }
+
+        /// <summary>
+        /// Returns a dictionary of query string parameters that represent this request. Only values that
+        /// do not have default values are in the dictionary. If everything is default, this returns an
+        /// empty dictionary.
+        /// </summary>
+        /// <returns></returns>
+        public override IDictionary<string, string> ToParametersDictionary()
+        {
+            var dictionary = base.ToParametersDictionary();
+            Debug.Assert(dictionary != null, "Base implementation is wrong. Dictionary should never be null");
+            if (!Milestone.IsBlank())
+            {
+                dictionary.Add("milestone", Milestone);
+            }
+            return dictionary;
+        }
+    }
+}