Add null checks and fix CodeAnalysis errors.

I enabled code analysis to the Octokit.Reactive project and
needed to fix things up.

Octokit/Helpers/StringExtensions.cs

@@ -19,6 +19,8 @@ namespace Octokit
 
         public static Uri FormatUri(this string pattern, params object[] args)
         {
+            Ensure.ArgumentNotNullOrEmptyString(pattern, "pattern");
+
             return new Uri(string.Format(CultureInfo.InvariantCulture, pattern, args), UriKind.Relative);
         }