Add null checks and fix CodeAnalysis errors.

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

Octokit/Http/ApiConnection.cs

@@ -26,28 +26,39 @@ namespace Octokit.Http
 
         public async Task<T> Get(Uri endpoint)
         {
+            Ensure.ArgumentNotNull(endpoint, "endpoint");
+
             return await GetItem<T>(endpoint);
         }
 
         public async Task<TOther> GetItem<TOther>(Uri endpoint)
         {
+            Ensure.ArgumentNotNull(endpoint, "endpoint");
+
             var response = await Connection.GetAsync<TOther>(endpoint);
             return response.BodyAsObject;
         }
 
         public async Task<string> GetHtml(Uri endpoint)
         {
+            Ensure.ArgumentNotNull(endpoint, "endpoint");
+
             var response = await Connection.GetHtml(endpoint);
             return response.Body;
         }
 
         public async Task<IReadOnlyCollection<T>> GetAll(Uri endpoint)
         {
+            Ensure.ArgumentNotNull(endpoint, "endpoint");
+
             return await pagination.GetAllPages(async () => await GetPage(endpoint));
         }
 
         public async Task<T> Create(Uri endpoint, object data)
         {
+            Ensure.ArgumentNotNull(endpoint, "endpoint");
+            Ensure.ArgumentNotNull(data, "data");
+
             var response = await Connection.PostAsync<T>(endpoint, data);
 
             return response.BodyAsObject;
@@ -55,6 +66,9 @@ namespace Octokit.Http
 
         public async Task<T> Update(Uri endpoint, object data)
         {
+            Ensure.ArgumentNotNull(endpoint, "endpoint");
+            Ensure.ArgumentNotNull(data, "data");
+
             var response = await Connection.PatchAsync<T>(endpoint, data);
 
             return response.BodyAsObject;
@@ -62,11 +76,15 @@ namespace Octokit.Http
 
         public async Task Delete(Uri endpoint)
         {
+            Ensure.ArgumentNotNull(endpoint, "endpoint");
+            
             await Connection.DeleteAsync<T>(endpoint);
         }
 
         async Task<IReadOnlyPagedCollection<T>> GetPage(Uri endpoint)
         {
+            Ensure.ArgumentNotNull(endpoint, "endpoint");
+
             var response = await Connection.GetAsync<List<T>>(endpoint);
             return new ReadOnlyPagedCollection<T>(response, Connection);
         }