bug fix: add missing header param to url class

[tie624]

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The header param was missing from the URL class.

I got an error while trying to use the header param according to the docs: https://github.com/Homebrew/homebrew-cask/blob/master/doc/cask_language_reference/stanzas/url.md

This fixed the error for me.

I didn't write any tests since I couldn't find an existing spec file for this.

[miccal]

Looks good, thank you @tie624.

[MikeMcQuaid]

@miccal If you're 👍🏻 you can ✅ and merge as desired.

[MikeMcQuaid]

Thanks @tie624 and thanks @miccal for the review and merge.