fix: update should take a Partial<T> object, fixes #1329

[phra]

Checklist

• Issue number for this PR: database module typescript definition: update signature is wrong #1329 (required)
• Docs included?: no
• Test units included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

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[phra]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[phra]

until this PR is not merged, you can experience casting problems when using a strict configuration of typescript.

[jamesdaniels]

Thanks for the PR. Sorry to fall behind on merging, got a travel bug coming back from the Firebase Dev Summit.