support Headers on api defaults

[Xiphe]

as with #538, oazapfts uses Headers internally instead of a plain object.

Initially we expanded this also to the defaults but it turned out that by this one would need to typecheck if defaults.headers is an object or of type Headers before setting to it.

To fix #551 we rolled back to only allow CustomHeaders on Defaults

In the future we might want to add a feature flag to opt into having a Headers object pre-set on the defaults.