Fix header type inconsistency

[sunspikes]

There is a small inconsistency the way Decode:: splitMessage() behaves, the headers variable referenced in splitMessage is supposed to of type Headers always, but when splitMessage is called with an empty content headers will become an empty array. This breaks client libraries for example: https://github.com/zendframework/zend-mail/blob/master/src/Message.php#L566

• Are you fixing a bug?
  • Detail how the bug is invoked currently.
  • Detail the original, incorrect behavior.
  • Detail the new, expected behavior.
  • Base your feature on the master branch, and submit against that branch.
  • Add a regression test that demonstrates the bug, and proves the fix.
  • Add a CHANGELOG.md entry for the fix.

[michalbundyra]

Thanks, @sunspikes!