[typescript-fetch]Bugfix additional properties in multipart requests #1351

Job	Run time
Build Java Client JDK11 (samples/client/echo_api/java/apache-httpclient)	1m 31s
Build Java Client JDK11 (samples/client/echo_api/java/resteasy)	1m 17s
Build Java Client JDK11 (samples/client/echo_api/java/native)	1m 40s
Build Java Client JDK11 (samples/client/echo_api/java/resttemplate)	1m 25s
Build Java Client JDK11 (samples/client/echo_api/java/feign-gson)	1m 26s
	7m 19s

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