[CI] UpdateSettingsRequestStreamableTests mutate failure

[davidkyle]

The error appears to be that for certain seeds the mutate() function has no effect

UpdateSettingsRequestStreamableTests.testEqualsAndHashcode <<< FAILURES!
   > Throwable #1: java.lang.AssertionError: UpdateSettingsRequest mutation should not be equal to original
   > Expected: not <indices : [index-kcp],{"HJS":{"Gqj":{"KILj":{"wkCM":{"Fhk":"pw"}}}}}>
   >      but: was <indices : [index-kcp],{"HJS":{"Gqj":{"KILj":{"wkCM":{"Fhk":"pw"}}}}}>

Reproduces:

 ./gradlew :server:test \
  -Dtests.seed=E1A4DF226F82345F \
  -Dtests.class=org.elasticsearch.action.admin.indices.settings.put.UpdateSettingsRequestStreamableTests \
  -Dtests.method="testEqualsAndHashcode" \
  -Dtests.security.manager=true \
  -Dtests.locale=ar-TN \
  -Dtests.timezone=Europe/Sarajevo

https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+matrix-java-periodic/ES_BUILD_JAVA=java10,ES_RUNTIME_JAVA=java8fips,nodes=virtual&&linux/217/console

[elasticmachine]

Pinging @elastic/es-core-infra