[PSR-7] Clarify MessageInterface::getHeaders()

- Per [the mailing list](https://groups.google.com/d/msg/php-fig/H_NR7qctw-8/9jDffi4_j7cJ), clarified the return value of `getHeaders()` to indicate the structure of the returned array.
Merge pull request #347 from weierophinney/hotfix/http-get-headers-return

[PSR-7] Clarify MessageInterface::getHeaders() return value