[typescript-fetch]Bugfix additional properties in multipart requests #1043

Job	Run time
Build Java Client JDK8 (samples/client/echo_api/java/apache-httpclient)	1m 22s
Build Java Client JDK8 (samples/client/echo_api/java/resteasy)	47s
Build Java Client JDK8 (samples/client/echo_api/java/okhttp-gson)	1m 7s
Build Java Client JDK8 (samples/client/echo_api/java/feign-gson)	1m 2s
Build Java Client JDK8 (samples/client/echo_api/java/resttemplate)	1m 15s
	5m 33s

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