Doesn't support empty quotes

[JamesMessinger]

An empty pair of quotes (e.g. "") should pass an empty string argument. But string-argv currently passes the actual quote characters instead.

STEPS TO REPRODUCE

$ node
> var stringArgv = require('string-argv');
undefined
> stringArgv('git commit --message ""')

EXPECTED RESULTS

[ 'git', 'commit', '--message', '' ]        // <--- Last arg is an empty string

ACTUAL RESULTS

[ 'git', 'commit', '--message', '""' ]      // <--- Last arg is two quotes