Update System.Net.Http reference to use NuGet

[tillig]

Describe the Bug

When running on .NET 4.6.1, the Autofac.Integration.WebApi.Own assembly appears to reference System.Net.Http 4.0.0.0, but when run on .NET 4.7.1 the reference appears to be System.Net.Http 4.2.0.0 which doesn't exist. This causes runtime failures looking for a missing assembly.

This issue is based on this StackOverflow question.

Steps to Reproduce

Create a project targeting .NET 4.7.1 that uses Autofac.Integration.WebApi.Own 5.0.0. This generates build warnings:

System.IO.FileNotFoundException: Could not load file or assembly 'System.Net.Http, Version=4.2.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
File name: 'System.Net.Http, Version=4.2.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
   at Owin.AutofacWebApiAppBuilderExtensions.UseAutofacWebApi(IAppBuilder app, HttpConfiguration configuration)

Expected Behavior

The version of System.Net.Http should be found and the project should be built without warnings.

Additional Info

The solution, based on searching, appears to be switching the System.Net.Http reference from a framework/assembly reference to a package reference.