It causes unexpected result if arguments are the same as option name

This issue is quite similar to #471.

``` javascript
var program = require('commander');

program
  .option('-f, --foo', 'add some foo')
  .parse(['node', './test.js', 'foo']);

console.log(program.args);  // program.args = []
console.log(program.foo);   // program.foo = []

// Expected Result:
// program.args = ['foo']
// program.foo = undefined
```
