[2.0.0-beta4] Fix attributes merging

There are 2 levels in an attribute array:

• The attribute array has only string, integer, float, or (list) array as values
• The second level has only string, integer or float as values.

Example:

href: "/"
rel:
 - author 
 - creator
size: 4
class:
 - foo 
 - bar

12 HTML attributes expects a list as value according to https://html.spec.whatwg.org/#unordered-set-of-unique-space-separated-to...

Set of space-separated tokens:

• class
• ping

Unordered set of unique space-separated tokens:

• blocking
• for
• headers
• itemprop
• itemref
• itemtype
• rel

Ordered set of unique space-separated tokens:

• accesskey

Unordered set of unique space-separated tokens, ASCII case-insensitive:

• sandbox
• sizes

So, because it is a mess, I am proposing to follow what https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Template%... is doing:

    public function merge(Attribute $collection) {
        $merged_attributes = NestedArray::mergeDeep($this->toArray(), $collection->toArray());
        foreach ($merged_attributes as $name => $value) {
            $this->storage[$name] = $this->createAttributeValue($name, $value);
        }
        return $this;
    }

See: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Component%21Util...