More http improvements

* Rename some things in Http to avoid name conflicts in DU cases between request/response
* Add more headers
* Port integration tests from http.fs
More http improvements

* Rename some things in Http to avoid name conflicts in DU cases between request/response
* Add more headers
* Port integration tests from http.fs
* Allow to override the response encoding
* Added RequestStream and AsyncRequestStream methods to Http to allow accessing the response stream directly (closes #210)