#1107 Make RepositoryRequest properties nullable and add tests

Octokit/Models/Request/RequestParameters.cs

@@ -33,7 +33,7 @@ namespace Octokit
             return (from property in map
                     let value = property.GetValue(this)
                     let key = property.Key
-                    where !String.IsNullOrEmpty(value)
+                    where value != null
                     select new { key, value }).ToDictionary(kvp => kvp.key, kvp => kvp.value);
         }
 
@@ -49,6 +49,9 @@ namespace Octokit
             Justification = "GitHub API depends on lower case strings")]
         static Func<PropertyInfo, object, string> GetValueFunc(Type propertyType)
         {
+            // get underlying type if nullable
+            propertyType = Nullable.GetUnderlyingType(propertyType) ?? propertyType;
+
             if (typeof(IEnumerable<string>).IsAssignableFrom(propertyType))
             {
                 return (prop, value) =>