[PSR-7] Clarify allowed header values.

Per [discussion on the mailing list](https://groups.google.com/d/msg/php-fig/N53l9apa6K4/LgGL_AyU0lEJ), this patch clarifies that objects are allowed for header values SO LONG AS they can be cast to string (i.e., they must implement `__toString()`).
Merge pull request #343 from weierophinney/hotfix/psr7-allow-object-header-values

[PSR-7] Clarify allowed header values.