Handle quoted empty string argument.

[andyjbgm]

Trying to generate ssh keys:

keygen = local['ssh-keygen']
keygen('-t', 'rsa', '-N', '', '-f', 't_key.pub')

results in:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/plumbum/commands.py", line 219, in __call__
    return self.run(args, **kwargs)[1]
  File "/usr/lib/python2.7/site-packages/plumbum/commands.py", line 275, in run
    return run_proc(p, retcode, timeout)
  File "/usr/lib/python2.7/site-packages/plumbum/commands.py", line 173, in run_proc
    stdout, stderr)
plumbum.commands.ProcessExecutionError: Command line: ['/usr/bin/ssh-keygen', '-t', 'rsa', '-N', '-f', 't_key.pub']
Exit code: 1
Stdout:  | Too many arguments.

i.e. the empty single quoted passphrase for -N is ignored.

Whereas the subprocess equivalent works as expected.

>>> com = ['ssh-keygen', '-t', 'rsa', '-N', '', '-f', 't_key.pub']
>>> r = subprocess.Popen(com, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
>>> r.returncode
>>> (output, error) = r.communicate()
>>> r.returncode
0
>>> output
"Generating public/private rsa key pair.\nYour identification has been saved in t_key.pub.\nYour public key has been saved in t_key.pub.pub.\nThe key fingerprint is:\n16:28:94:a5:73:e0:56:1f:73:88:b1:86:e4:32:8a:28 andy@vmandy.getme\nThe key's randomart image is:\n+--[ RSA 2048]----+\n|    +o+oo..      |\n|   =.=.+.+       |\n|  o O = o        |\n|o. + =   .       |\n|E       S        |\n|.      .         |\n|                 |\n|                 |\n|                 |\n+-----------------+\n"