Make sure the broker is returned correctly

[dizzzz]

During this Mongrel logging, we are also seeing a /WARN (BrokerPool.java
[release]:1278) - Broker org.exist.storage.NativeBroker_exist_5 was returned
with extraneous Subjects, cleaning...
java.lang.IllegalStateException: DBBroker pushSubject/popSubject mismatch
.../ Do we have a problem?

Mongrel/java/src/org/exist/mongodb/shared/ContentSerializer.java

Line 97 in bb9e621

final Serializer serializer = context.getBroker().newSerializer();