Fixed exception in ASP.NET WebApi due to missing method

This fix enables the latest ASP.NET WebApi from NuGet to work on Mono 3.x.

All I did was adding a method stub for `GetBufferlessInputStream()` in System.Web.HttpRequest (which is new in .NET 4.0).

It currently just throws a NotImplementedException as WebApi doesn't use this method by default (you'd need to explicitly enable bufferless input in config), but an exception occured when instantiating an Api-Controller even though the method wasn't used.

Before applying the fix (using curl to query the api):
![before.png](https://f.cloud.github.com/assets/1376924/140331/22a9c600-721c-11e2-801a-462ba72cec21.png)

After:
![after.png](https://f.cloud.github.com/assets/1376924/140332/28e57a6e-721c-11e2-9756-b36f950e5b82.png)