fix createWriteStream failing to create new files, fixes #428

[dy-dx]

This issue provides most of the context: https://github.com/streamich/unionfs/issues/428

#428 suggests just to replace fs.statSync(path) with fs.statSync(dirname(path)), but that could cause surprising behavior in the case where multiple filesystems include the parent directory.

I've added a regression test for this case: "createWriteStream writes to an existing file even if parent dir exists on an earlier fss"

[dy-dx]

I've just noticed that unionfs iterates backwards through the this.fss array for most operations, except when using createWriteStream/createReadStream.

This seems like a mistake, and I'll address it in ~this PR as well~.

Addressed in https://github.com/streamich/unionfs/pull/785

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