mustache: fix filter application for children. #99

Job	Run time
test-typescript-ccda-example (latest)	26s
test-csharp-sdk (latest, 8.0.x)	1m 20s
test-mustache-java-r4-example (latest, 25)	1m 3s
test-mustache-java-r4-example (latest, 21)	1m 7s
test-typescript-sdk-r4 (latest)	17s
	4m 13s

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