Add has() method to ItemInterface

I think that:

• DynamicItem needs to check if the requested field exists on the $data array using array_key_exists();
• OpmlItem, SitemapItem and SyndicationItem should return TRUE when the requested field is a defined property on the class. For example, SyndicationItem has a property called $author_name, so that would mean that $item->has('author_name'); is always TRUE. I think it would need to be done that way because I don't know how else you could distinguish NULL from "doesn't exist" for these items. I think that class properties that aren't set will just equal to NULL, or isn't this the case?
• BaseItem could already implement has() that would check if the class property exists. However, DynamicItem should not return TRUE for $item->has('data'); (unless the data array contains an array key called "data").

This would result into at least the following test cases:

• SyndicationItem: $item->has('author_name'); should return TRUE.
• SyndicationItem: $item->has('foo'); should return FALSE.
• DynamicItem with fields 'foo' and 'bar' set: $item->has('data'); should return FALSE.
• DynamicItem with fields 'foo' and 'bar' set: $item->has('foo'); should return TRUE.
• DynamicItem with fields 'foo' and 'bar' set: $item->has('qux'); should return FALSE.
• DynamicItem with field 'data' set: $item->has('data'); should return TRUE.

Perhaps for OpmlItem and SitemapItem we need similar tests (almost the same) as for SyndicationItem.

Related issue that also affects the implementation of item classes: 🐛 Creation of dynamic property Drupal\feeds\Feeds\Item\SyndicationItem::$parent:fid is deprecated Needs work .