parameter is nil when single dash argument provided

For some reason a method with a non-defaulted parameter on the end receives nil when a single dash is given for the last argument on the command line.

Example:

```
def test(a, b, c)
  puts "a=#{a}, b=#{b}, c=#{c}"
end
```

Command Line:

```
test 1 2 -
```

Expected:

```
"a=1, b=2, c=-"
```

Actual:

```
"a=1, b=2, c=nil"
```
