Fixed exception caused by using preserve_quotes argument of with_argument_list decorator

[kmvanbrunt]

Fixes #633

[codecov]

Codecov Report

Merging #634 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #634      +/-   ##
==========================================
+ Coverage   94.08%   94.08%   +<.01%     
==========================================
  Files          10       10              
  Lines        2957     2959       +2     
==========================================
+ Hits         2782     2784       +2     
  Misses        175      175

Impacted Files	Coverage Δ
cmd2/cmd2.py	94.52% <100%> (ø)	⬆️

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[tleonhardt]

Thanks for finding and fixing this bug. Shame on me for not creating a relevant unit test in the first place!

Your changes look good, but please take a look at my comments.

Feedback from @kotfu would also be appreciated - he may have an interesting decorator trick up his sleeve that could get around the required parentheses.

[tleonhardt]

I'd recommend keeping the relevant comment in the docs and just updating it accordingly.

If we can find some intelligent way of not requiring the parentheses, then that would be great.

[tleonhardt]

I would recommend keeping this note and just updating it to show the required parentheses.

[tleonhardt]

See this StackOverflow post for a possible way we could use the decorator with or without parents: https://stackoverflow.com/questions/35572663/using-python-decorator-with-or-without-parentheses