[5.3] Resolve issues with multi-value select elements

[leewillis77]

This PR implements @spikerok's suggestion from #10514 (comment) to strip [] out of element names.

I've been running with this for a week or so and everything looks OK.

---

Closes #10514.

[taylorotwell]

Is it possible to have a test or something for this?

[themsaid]

Here's a complete test case:

<form action="/" method="post">
    {{csrf_field()}}
    <select name="hills[]" multiple>
        <optgroup value="Testing">
            <option value="1">Testing 1</option>
            <option value="2">Testing 2</option>
        </optgroup>
    </select>

    <button>Submit</button>
</form>

Doing:

public function testBasicExample()
{
    $this->visit('/');

    $this->select([1,2], 'hills[]')
    ->press('Submit');
}

Will throw an error Unreachable field from Symfony\Component\DomCrawler\FormFieldRegistry

[iWader]

I'm having some issues with the Unreachable field exceptions I keep getting.

Is it possible to test a form with a multiple select?