fix os.copy with mergeFolders=true

[pawelsadlo]

Bug appears when we call os.copy with mergeFolders = true

if target doesn't contain inner directories corresponding to source os.copy with mergeFolders will fail try to copy source directory as it was a file and fail with FileAlreadyExistsException

createFolders = true doesn't fix things

this is because os.isDir(targetDir) returns false if path doesn't exist

This PR adds explicit target directory creation if sourceDirectory is directory and targetDirectory doesn't exist

[lefou]

Can you please add a test for the case you described?

[pawelsadlo]

@lefou Never mind, I misunderstood replaceExisting parameter behaviour (thought it would simply omit coliding files but its actually throwing exception)