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

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:

- Keys MUST be header names
- Values MUST be arrays of strings