Fix File.flush return value for case of zero bytes to flush.

[jemc]

Currently, File.flush returns false when the number of bytes pending to write is zero, even though the docstring says that:

    Returns false if the file wasn't opened with write permission.
    Raises an error if not all the bytes were written.
    Returns true if it sent all pending data.

This situation should fall under the true condition - all pending data was successfully written - it's just that the number of bytes was zero. It definitely doesn't fall under the false condition because it has nothing to do with write permissions.

This PR fixes that bug and adds a test.

[jemc]

Let's try a different approach in the test and see if it fixes the windows build...

[mfelsche]

What was the actual windows issue?

[jemc]

I was testing file size after the write, expecting 7 bytes for "foobar\n" and got 8 bytes. Maybe a line endings issue with \r somehow being added?