Fix the popen with "w" by closing the writer (otherwise wait never terminates)

[scr-oath]

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Changes proposed in this pull request:

• When closing a lFileProcess close the writer before calling Wait
• Normalize the stdout closing to also use casting to .(io.Closer)
• Move the close of stdout after the Wait - data could still be written before the process finishes.
• Add a test for read and write

[scr-oath]

@yuin any chance you could merge this fix to the close method so that processes that are opened to be sent information (on their stdin) can be awaited too?