Add phpdoc to MessageInterface::getHeaders(), consider not using null

[samdark]

Currently, MessageInterface::getHeaders() doesn't have phpdoc. We should add it.

Also, it may return null which means "do not add headers". An empty array could be possibly used for the same purpose. Need to consider it.

[vjik]

Also, it may return null which means "do not add headers". An empty array could be possibly used for the same purpose. Need to consider it.

null — headers don't set. Later, in mailer we can add default headers.
[] — headers is set. Later, in mailer we don't add default headers.

[samdark]

Alright. Then we need phpdoc only.