Ensure that CancellationToken is propagated to built-in request content types (dotnet...

Ensure that CancellationToken is propagated to built-in request content types (dotnet/corefx#39474)

* Ensure that CancellationToken is propagated to built-in request content types

We don't currently expose the overload that would allow CancellationToken to be accessed by HttpContent-derived types in general, but we can at least ensure that when using the built-in content types, the CancellationToken provided to SendAsync is appropriately threaded through.  We were doing this previously in only a few cases, where we knew that the previous overload wouldn't be overridden (namely internal types and sealed types), but we can enable the 90% scenario as well by doing a type check at run-time.

* Disable cancellation test for UAP handler

Commit migrated from https://github.com/dotnet/corefx/commit/450f49a1a80663529b31d3defafbd5e59822a16a