Make header splitting more consistent and less buggy

[weierophinney]

This patch covers related bugs/issues from zendframework/zend-mime#40

Unit tests added for 3 different cases

---

Originally posted by @ozanakman at zendframework/zend-mime#41

[weierophinney]

This package is considered feature-complete, and is now in security-only maintenance mode, following a decision by the Technical Steering Committee.
If you have a security issue, please follow our security reporting guidelines.
If you wish to take on the role of maintainer, please nominate yourself

If you are looking for an actively maintained package alternative, we recommend:

• symfony/mime