Add ability to send headers through repositories

[rapkis]

Description

Accept a new array $headers = [] argument for every method used in repositories (default to an empty array for backward compatibility). Then forward those arguments to the underlying DocumentClient class, which already accepts them.

Motivation and context

Closes https://github.com/swisnl/json-api-client/issues/102

How has this been tested?

Tested the headers inputs in RepositoryTest

Screenshots (if appropriate)

Types of changes

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• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

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[rapkis]

By the way, I've noticed that there's an unrelated styling issue in UtilTest.php (caught by composer check-style), but didn't want to make any unrelated changes. Let me know if you'd like me to, or if that should be solved separately.

[bbrala]

Thanks for the pr. A thought, shouldn't there be a test with and without a header?