* Fix complexity for DefaultHttpHeaderMapper
* Deprecate `DefaultHttpHeaderMapper` constants which are fully
Spring Web `HttpHeaders` constants
* Introduce lowercase constants for `getHttpHeader()` switch
* Reuse `HttpHeaders` API as much as possible
* Simplify logic in some methods to break them to smaller methods
* * Remove Java Streams from critical paths
* Rework `setHttpHeader()` for `switch`
* * Use `ObjectUtils.toObjectArray()` to simplify complexity
* * Fix complexity in `setAccept()`, `setAcceptCharset()`, `setAllow()`
& `setIfNoneMatch()` using newly extracted `valueToCollection()`
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