feat(headers) Add Access-Control-Allow-Credentials header

[brycefisher]

There is only one acceptable value for this header, "true", so
there header struct is unit-like (no fields nor data inside). See
documentation and tests for more info.

Closes #655

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[brycefisher]

😭 I forgot the colon.... sinc Ryman just suggested an improvement over my impl, I'm going to close and fix up this commit with his suggestions once the tests pass.