Fixed implementation of conversion to std::fs::File

[michaelvanstraten]

The current implementation of the From trait for PipeReader and PipeWriter for std::fs::File is wrong, resulting in a invalid handle error whenever trying to use the produced file.

Rust makes it kind of hard to see why this is wrong, but I added a comment to explain the issue.

[zhiburt]

Hi @michaelvanstraten

Good catch indeed;

Drop of PipeReader/PipeWriter is called before we return a file and we close File Handle because of.

Thanks.
Take care.