Base classes need protected setters, ActivityPayload had privates (#2591)

Octokit.Tests/Models/CheckSuiteEventTests.cs

@@ -18,6 +18,9 @@ namespace Octokit.Tests.Models
             Assert.Equal("rerequested", payload.Action);
             Assert.Equal("73955d02043135d48809add98052c2170522158f", payload.CheckSuite.HeadSha);
             Assert.Equal(CheckStatus.Queued, payload.CheckSuite.Status.Value);
+            Assert.NotNull(payload.Installation);
+            Assert.NotNull(payload.Repository);
+            Assert.NotNull(payload.Sender);
         }
     }
 }

Octokit/Models/Response/ActivityPayloads/ActivityPayload.cs

@@ -15,9 +15,9 @@ namespace Octokit
             Installation = installation;
         }
 
-        public Repository Repository { get; private set; }
-        public User Sender { get; private set; }
-        public InstallationId Installation { get; private set; }
+        public Repository Repository { get; protected set; }
+        public User Sender { get; protected set; }
+        public InstallationId Installation { get; protected set; }
 
         internal string DebuggerDisplay
         {