Add a new constructor and an accessor to `Upgrade`#105

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fasterthanlime wants to merge 2 commits intohyperium:masterfrom fasterthanlime:upgrade-accessor

Commits on Mar 15, 2022

Add a new constructor and an accessor to Upgrade
fasterthanlime
committed
Only provide h2c constructor, return an Option<&str> rather than a HeaderValue
fasterthanlime
committed