The subprocess.Popen function reorganises the command line for process
creation when shell=True is passed in under Windows. It runs the
existing executable & arguments as arguments to %COMSPEC% /c. However
this fails when a second parameter (typically a filename) has an
embedded space. The patch attached adds extra parameters in this case
and adds appropriate unit tests.

Frankly I'm new to unittests and I more-or-less cloned an existing one
to make this work. Happy to be corrected if there's things done wrong &c.