Mike Hadlow has posted a little console app called AsmSpy that rather nicely lists each assembly's references:
Reference: System.Net.Http.Formatting
4.0.0.0 by Shared.MessageStack
4.0.0.0 by System.Web.Http
Reference: System.Net.Http
2.0.0.0 by Shared.MessageStack
2.0.0.0 by System.Net.Http.Formatting
4.0.0.0 by System.Net.Http.WebRequest
2.0.0.0 by System.Web.Http.Common
2.0.0.0 by System.Web.Http
2.0.0.0 by System.Web.Http.WebHost
This is a much quicker way to get to the bottom of the warning MSB3247, than to depend on the MSBuild output.