Serialize null in positional arrays

[scaytrase]

Fix #571

[scaytrase]

I'm ready to fix some related tests but I'm not sure which failures belong to the patch as the whole build history is red
https://travis-ci.org/schmittjoh/serializer/builds

[maennchen]

@scaytrase You can now rebase on the original master and everything should get green.

[scaytrase]

Thanks, all green.

[scaytrase]

The given commit actually does not resolve the problem.

It fails a simple test

    public function testNullArraySerialization()
    {
        $context = SerializationContext::create();
        $context->setSerializeNull(true);
        $array = [0, null, 'a'];
        self::assertSame($array, $this->serializer->toArray($array, $context));
    }

How we force an array being normalized as array, not object?

Failed asserting that Array &0 (
    0 => 0
    2 => 'a'
) is identical to Array &0 (
    0 => 0
    1 => null
    2 => 'a'
).