Why does the test pass in go1.12 but fails in g1.13 due to "bad handshake?"

Question

I am working with/on an open-source project (https://github.com/taskcluster/websocktunnel). After running go get -u github.com/taskcluster/websocktunnel and cding into the directory and running go test ./... the test fails. Although it passes in go1.12. I have read the release notes on go1.13 but I cannot find anything that leads to a bad handshake. Maybe the solution might be obvious to someone with more networking experience.

Thanks in advance!