Use the dictionary for 404

Cleaning up the code to use the dictionary lookup.

Octokit/Http/Connection.cs

@@ -281,6 +281,7 @@ namespace Octokit
             {
                 { HttpStatusCode.Unauthorized, GetExceptionForUnauthorized },
                 { HttpStatusCode.Forbidden, GetExceptionForForbidden },
+                { HttpStatusCode.NotFound, response => new NotFoundException(response) },
                 { (HttpStatusCode)422, response => new ApiValidationException(response) }
             };
 
@@ -293,11 +294,6 @@ namespace Octokit
                 throw exceptionFunc(response);
             }
 
-            if ((int)response.StatusCode == 404)
-            {
-                throw new NotFoundException(response);
-            }
-
             if ((int)response.StatusCode >= 400)
             {
                 throw new ApiException(response);