Replace magic literals with class constants in SecurityHeadersMiddleware
Replace magic literals with class constants in SecurityHeadersMiddleware

Except for the HTTP header keys. They could be constants, too, but should be placed in another class, e.g. general HTTP related.

Constant doc blocks as per PSR-19 (in draft)
https://github.com/php-fig/fig-standards/blob/master/proposed/phpdoc-tags.md#522-var

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