Solve arguments() showing empty keys when exsiting short-only option

[ldeng-ustc]

Solve #317

Before this PR:

$ ./example --list -x 1        
list = true
 = 1            # expect "x = 1" here
range = false (default)
compile = false (default)
...

After:

$ ./example --list -x 1        
list = true
x = 1          # OK
range = false (default)
compile = false (default)

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This change is 

[jarro2783]

Thanks, can you write a unit test?

[ldeng-ustc]

I add check on arguments() key and value in short option unit test, and write a new unit test for iterator.

[jarro2783]

Thanks for your PR.