various HttpSerialiser modifications
various HttpSerialiser modifications

1. Revise IsClosed (now virtual, with protected setter)
2. Expand on documentation for GetStream (now notes that the serialiser
   is closed upon calling)
3. Define abstract method Flush()
4. Mark Close(bool) virtual, and provide a default method body. Now
   sets IsClosed to true and conditionally calls Flush. No action if the
   serialiser is already closed.

McSherry/Zener