Fix parsing array arguments with :into option

[fatkodima]

Fixes #72.

When we defined an array parameter and use :into when parsing, when parsed its value (array) incorrectly gets destructured in

optparse/lib/optparse.rb

Line 1773 in 1211f70

callback!(setter, 2, sw.switch_name, *val) if setter

and passed to the callback (aka setter) in

optparse/lib/optparse.rb

Line 1702 in 1211f70

setter = ->(name, val) {into[name.to_sym] = val} if into

and so we get the ArgumentError.