replace object.assign by util.assign to make it work in IE

[borisnieuwenhuis]

Fixes #1642

TODO:

• If this PR is a new feature, 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
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change
• Add your name and email to the AUTHORS file (optional)
• If this was a change that affects the external API, update the docs and post a link to the PR in the discussion

[apollo-cla]

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[helfer]

Thanks @borisnieuwenhuis, could you make sure the tests pass and also add a changelog entry? 🙂

[borisnieuwenhuis]

@helfer looks like all test are passing now.
it is my first typescript so I hope it is sort of ok :-)

[helfer]

Looks great, thanks a lot @borisnieuwenhuis !