fix: add httpEquiv to additionalMetaTags

[kahoowkh]

Description of Change(s):

Add httpEquiv to additionalMetaTags

Close #535

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• Have you updated the documentation to go with your changes? Yes
• Have you written or updated unit tests? Yes
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https://github.com/garmeeh/next-seo/blob/master/CONTRIBUTING.md

[garmeeh]

Thanks for the contribution 😄

[garmeeh]

@all-contributors add @kahoowkh doc code test

[allcontributors]

@garmeeh

I've put up a pull request to add @kahoowkh! 🎉