Fix InMemory not overwriting on move

[kynx]

This fixes the InMemoryFilesystemAdapter not copying or moving directories

[frankdejonge]

Can you make this strategy configurable via a constructor option? Otherwise this would be a major BC break and I do not want to cause that.

[kynx]

@frankdejonge Uh, OK. Guess the documentation should be updated to reflect the fact that not all adapters behave the same?

[kynx]

@frankdejonge Actually I might have got a little carried away with copying and moving directories. My initial problem was with the SFTP adapter not overwriting files when moving after upgrading an old project from 1.x to 3.x. Looking back at the docs there is no mention of copying or moving directories.

I do think it's a bug that some adapters don't overwrite files on move - the docs clearly say they do. If I moved the directory stuff to a different PR targeting the next major, would that work?

[kynx]

@frankdejonge I've removed the directory copy / move stuff. This now just brings this adapter in line with the others, overwriting existing files on move.

Please let me know if you still think this needs a constructor arg for specifying this behaviour.