Introducing a configurable normalization strategy
for the HeaderDataCollection
New tests and some renaming
Making the `normalizeKey()` method protected,
there's no reason that it should be part of our public API
More tests for more coverage
Deprecating the old header normalization method
Make tests compatible with the deprecation
Merge pull request #259 from Rican7/feature/configurable-header-normalization

Feature - Configurable header normalization
Changing our callback data-structures and iterations
over to use the SplStack and SplQueue data-structures
More properly calling `enqueue()` for the SplQueue