Error when argument has same name as a flag

`a.js`:
```js
const c = require("commander");
c
    .option("--foo [value]")
    .parse(process.argv);
console.log({ foo: c.foo, args: c.args });
```
Usage:
```sh
$ node a.js arg --foo value
{ foo: 'value', args: [ 'arg' ] }
$ node a.js foo --foo value
{ foo: [], args: [] }
```

Expected `{ foo: 'value', args: [ 'foo' ] }`.