refactoring ApiConnection.EmptyBody to RequestBody.Empty to break the inverse dependency from Connection to ApiConnection

Octokit/Http/RequestBody.cs

@@ -0,0 +1,12 @@
+namespace Octokit
+{
+    /// <summary>
+    /// Container for the static <see cref="Empty"/> method that represents an
+    /// intentional empty request body to avoid overloading <code>null</code>.
+    /// </summary>
+    public static class RequestBody
+    {
+        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2211:NonConstantFieldsShouldNotBeVisible")]
+        public static object Empty = new object();
+    }
+}