Change ResponseEntityWriterContributor to allow custom contributor to be...

... injected between the Knowstages.IResponseEncoding and the end of the PipelineContinuation.
I've just tested without the .And.Before<KnownStages.IEnd>() on the ResponseEntityWriterContributor, and you are right. The "EndContributor: IEnd" is still being called as well as being the last one to be called. All other contributors (including the IResponseEncoding) are being executed before as expected. 

So we can get rid of this indeed and just keep the fix for "PipelineContinuation.Finished" => "PipelineContinuation.Continue".

What do you think ?