Commented Feature: Add overloads taking a CancellationToken parameter [983]

Currently some of TAP-based methods in the aspnetwebstack libraries does not have overloads with _CancellationToken_ parameter. For instance following classes:

_System.Net.Http_: HttpContent and successors.
```
HttpContent: ReadAsStringAsync, ReadAsByteArrayAsync, ReadAsStreamAsync, etc.
```
_System.Net.Http.Formatting_: HttpContent extensions.
```
HttpContentMessageExtensions: ReadAsHttpRequestMessageAsync
HttpContentMultipartExtensions: ReadAsMultipartAsync
HttpContentFormDataExtensions: ReadAsFormDataAsync
HttpContentMessageExtensions: ReadAsHttpRequestMessageAsync
MultipartStreamProvider: ExecutePostProcessingAsync
```
_System.Net.Http.Formatting_: MediaTypeFormatter and successors.
```
MediaTypeFormatter: ReadFromStreamAsync, WriteToStreamAsync
BufferedMediaTypeFormatter
FormUrlEncodedMediaTypeFormatter
JsonMediaTypeFormatter
XmlMediaTypeFormatter
```
_System.Web.Http_: Model binder which breaks cancellable pipeline.
```
FormatterParameterBinding: ReadContentAsync
```
Methods of above classes potentially may be executed for a long time period. In addition if we're trying to create custom _MediaTypeFormatter_ we unable to cancel _ReadFromStreamAsync_ and _WriteToStreamAsync_ methods.

I suppose that it will be great if such methods will contain an overloads with a _CancellationToken_ parameter, which should be properly integrated into Web API message processing pipeline.

Code sample how it can be refactored: https://aspnetwebstack.codeplex.com/SourceControl/network/forks/dtretyakov/CancellationToken/contribution/4472

P.S. Discussions looks dead - no answers for over 2 weeks: http://aspnetwebstack.codeplex.com/discussions/438601
Comments: [Fixed](https://aspnetwebstack.codeplex.com/SourceControl/changeset/3e71a6358d5dcaeb40825cf0bfb7c3e92b2b50af).