[PropertyAccess] Fix setting public property on a class having a magic getter

[lolautruche]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	N/A
License	MIT
Doc PR	N/A

When using PropertyAccessor with an object having both a public property
and a magic getter, and one wants to update this property,
PropertyAccessor may lose the property reference.
This occurs when the public property value is a hash:

class Foo
{
    /**
     * Example: $this->someProperty['foo']['bar'] = 'baz'
     * @var array
     */
    public $someProperty;

    public function __get($name)
    {
        // ...
    }
}

$obj = new Foo();
$obj->someProperty = ['foo' => ['bar' => 'some_value']];

$propertyAccessor->setValue($obj, 'someProperty[foo][bar]', 'another_value');

echo $obj->someProperty['foo']['bar'];
// Before this patch: 'some_value' => fail
// After this patch: 'another_value' => correct

Furthermore, public properties are always used before __get() by PHP.

This bug is visible since v2.6.5 as d733a88 changed the way
setValue() works.

[lolautruche]

Target branch is 2.6 (2.5 is not affected)

[lolautruche]

Travis fail doesn't seem to be caused by my PR (unrelated deprecation warnings)

[lolautruche]

Closing to reopen against 2.6 branch: #14605