Support requests with HttpContent body

If a 3rd party client needs to provide a specific HttpContent, we should
allow that in the adapter. Our clients probably shouldn't do this as it
would break encapsulation.

Octokit.Tests/Exceptions/ApiValidationExceptionTests.cs

@@ -3,7 +3,6 @@ using System.IO;
 using System.Linq;
 using System.Runtime.Serialization.Formatters.Binary;
 using NSubstitute;
-using Octokit.Internal;
 using Xunit;
 using Xunit.Extensions;