[CI] SearchFieldsIT testDocValueFields failure

[martijnvg]

Failure:

09:47:47    > Throwable #1: java.lang.ClassCastException: org.joda.time.MutableDateTime cannot be cast to org.joda.time.DateTime
09:47:47    > 	at __randomizedtesting.SeedInfo.seed([46906270EEFFE0CB:CA6A1FC86A640665]:0)
09:47:47    > 	at org.elasticsearch.search.fields.SearchFieldsIT.testDocValueFields(SearchFieldsIT.java:806)
09:47:47    > 	at java.lang.Thread.run(Thread.java:748)

Failure reproduces:

./gradlew :server:integTest -Dtests.seed=46906270EEFFE0CB -Dtests.class=org.elasticsearch.search.fields.SearchFieldsIT -Dtests.method="testDocValueFields" -Dtests.security.manager=true -Dtests.locale=is-IS -Dtests.timezone=Asia/Jerusalem
09:47:47   1> [2018-07-12T12:47:47,113][INFO ][o.e.c.m.MetaDataDeleteIndexService] [node_sm0] [test/j69M4xO1Qvq58L5XmKFb

Build url: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+intake/2305/console

[elasticmachine]

Pinging @elastic/es-search-aggs