Add type hint PolicyInterface to HeaderInterface
Fix whitespace: blank lines and tabs
Inherit from ContentSecurityPolicyReportOnlyHeader
Improve multiline indentation in tests
Add HeaderList for holding multiple headers

- New interface ToArrayInterface.
- New class HeaderList inheriting RenderableList and implementing ToArrayInterface.
Put constants on the left of comparisons

Comply with the Symfony convention:

- http://symfony.com/doc/current/contributing/code/standards.html
- https://github.com/symfony/symfony-docs/commit/50f3acd2f24557125d1bb4dc18f07a1dbae23f52#commitcomment-5152922