bugfix: Make sure the PDO and Serialize metadata storage handlers return...

bugfix: Make sure the PDO and Serialize metadata storage handlers return metadata containing the 'entityid' key.

While investigating issue #393, we noticed that these two handlers don't enforce the entity ID of each entity to be set in the 'entityid' key of the metadata array (the Flatfile and XML handlers to enforce this). Since this is the way we propagate the entity ID (code using metadata won't necessarily have access to the key of the metadata array containing the entity ID, or such a thing might not even exist), we need to make sure the entity ID is set inside the array. Otherwise, if the metadata for an entity does not contain it directly, the 'ExpectedIssuer' would be set to null, an error recorded, and authentication would fail as the entity ID of the IdP issuing an assertion would not match.