fs.WriteStream: Handle modifications to fs.open

If the fs.open method is modified via AOP-style extension, in between
the creation of an fs.WriteStream and the processing of its action
queue, then the test of whether or not the method === fs.open will fail,
because fs.open has been replaced.

The solution is to save a reference to fs.open on the stream itself when
the action is placed in the queue.

This fixes isaacs/node-graceful-fs#6.