fix: preserve existing blob hashes when 'X-Goog-Hash header' is not present

[MartinKilonzo]

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Fixes #266 🦕

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[google-cla]

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[google-cla]

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[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[MartinKilonzo]

Thanks for the review!
Added the proposed changes. @tseaver, would you mind checking your CLA per @googlebot:

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[tseaver]

@googlebot I fixed it

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All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

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[tseaver]

@googlebot I consent

[google-cla]

CLAs look good, thanks!

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[google-cla]

☹️ Sorry, but only Googlers may change the label cla: yes.

[tseaver]

@MartinKilonzo Thanks for persevering!