Only one header value of several is sent with MultipartForm body #1222

Job	Run time
IT / GraphQLWS-Tests (18.13.0)	53s
IT / SockJS-WriteHandler-Tests (16.13.1)	1m 28s
CI (windows-latest, 11) / Run tests	5m 6s
CI (ubuntu-latest, 21) / Run tests	6m 5s
CI (ubuntu-latest, 11) / Run tests	6m 5s
Deploy	0s
	19m 37s

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