Commented Issue: Add Set* extension methods for all HttpRequestMessage properties [156]

For unit testing you often need to setup a full HttpRequestMessage so that conneg can be run in the API controller action. We have Get* extension methods for the properties hanging off of HttpRequestMessage.Properties but we don't hvae Set* methods. We should add the Set* methods.
Comments: BTW, let us not make those keys internal or obsolete. There is no good reason to break existing app here.