_I.e._ fixed:

    (cat << "meow") & FG

Modifiers writing to stdout -- _e.g._: FG, TEE, RETCODE(FG=True) and
TF(FG=True) -- invoke commands with the specification `stdin=None`.
This is handled correctly by BaseCommand; however, StdinDataRedirection
misinterprets this as a further attempt to specify stdin (_i.e._ to
specify _something_ rather than _nothing_).

This change brings StdinDataRedirection in line with its base class,
ignoring `None` as well as `PIPE`.

(Though a technical detail, the class would also have failed any
invocation setting `stdin=PIPE`, along the lines of: "TypeError: got
multiple values for keyword argument 'stdin'". The kwargs passed to the
wrapped command's `popen` are corrected as well.)

resolves tomerfiliba#604

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>