adding checks for json end_document in http transport

[duzinkie]

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the issue is discussed in: https://github.com/apollographql/apollo-kotlin/issues/5885

this is the recommended fix for the no http batching scenario, for the main branch

[apollo-cla]

@duzinkie: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

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[martinbonnin]

Hi @duzinkie 👋 . Want to get this through the finish line?

[duzinkie]

Hey @martinbonnin - sorry for the delays, I was on vacation - will be catching up on this this week.