Add null checks and fix CodeAnalysis errors.

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

Octokit/Helpers/CollectionExtensions.cs

@@ -6,6 +6,8 @@ namespace Octokit
     {
         public static TValue SafeGet<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> dictionary, TKey key)
         {
+            Ensure.ArgumentNotNull(dictionary, "dictionary");
+
             TValue value;
             return dictionary.TryGetValue(key, out value) ? value : default(TValue);
         }