<!-- Thanks for filing a pull request on Apollo Client! A few automated bots may chime in on your PR. They are here to help with reviewing and ensuring Apollo Client is production ready after each pull request merge. - apollo-cla will respond asking you to sign the CLA if this is your first PR. It may also respond with warnings, messages, or fail the build if something is off. Don't worry, it'll help you to fix what is broken! - bundlesize is a status check to keep the footprint of Apollo Client as small as possible. - circleci will run tests, checking style of code, and generally make sure everything is working as expected Please look at the following checklist to ensure that your PR can be accepted quickly: --> Adds a changelog entry and documentation for the new `preserveHeaderCase` option from https://github.com/apollographql/apollo-client/pull/9891 ### Checklist: - ~~[ ] If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)~~ - ~~[ ] Make sure all of the significant new logic is covered by tests~~