Reference Middleware and Strategy Headers via static keyword instead of self

[jblz]

Classes CacheMiddleware & GreedyCacheStrategy are shipped with this package with specific header names set via class constant:

guzzle-cache-middleware/src/CacheMiddleware.php

Lines 21 to 26 in 7f25853

	const HEADER_RE_VALIDATION = 'X-Kevinrob-GuzzleCache-ReValidation';
	const HEADER_INVALIDATION = 'X-Kevinrob-GuzzleCache-Invalidation';
	const HEADER_CACHE_INFO = 'X-Kevinrob-Cache';
	const HEADER_CACHE_HIT = 'HIT';
	const HEADER_CACHE_MISS = 'MISS';
	const HEADER_CACHE_STALE = 'STALE';

guzzle-cache-middleware/src/Strategy/GreedyCacheStrategy.php

Line 22 in 7f25853

const HEADER_TTL = 'X-Kevinrob-GuzzleCache-TTL';

Extending these classes does not currently permit altering those values (unless the functionality altered in this PR are also overridden).

This change updates the references to these class constants to use the static keyword as opposed to self to take advantage of PHP's Late Static Bindings feature.

That way, custom implementations values are prioritized above the defaults for these headers.

I ran the automated test suite and it still passed for me.

[Kevinrob]

Thanks!