Fix index set overview with closed indices

[bernd]

The code was trying to modify an immutable map with details for closed indices.

Fixes the index overview page when closed indices are present.

2017-06-22 13:53:18,603 ERROR: org.graylog2.shared.rest.exceptionmappers.AnyExceptionClassMapper - Unhandled exception in REST resource
java.lang.UnsupportedOperationException: null
    at com.google.common.collect.ImmutableMap.put(ImmutableMap.java:473) ~[graylog.jar:?]
    at org.graylog2.rest.resources.system.indexer.IndexerOverviewResource.lambda$getIndexerOverview$1(IndexerOverviewResource.java:128) ~[graylog.jar:?]
    at java.lang.Iterable.forEach(Iterable.java:75) ~[?:1.8.0_131]
    at org.graylog2.rest.resources.system.indexer.IndexerOverviewResource.getIndexerOverview(IndexerOverviewResource.java:128) ~[graylog.jar:?]
    at org.graylog2.rest.resources.system.indexer.IndexerOverviewResource.index(IndexerOverviewResource.java:114) ~[graylog.jar:?]