MapReduce on custom classes doesn't work in case of CacheLoader

The issue described in ISPN-2138 reappears in case when the cache is configured with CacheLoader.
The ClassCastException is thrown when MapReduce processes the entries from CacheLoader.

You can find the test which reproduces / verifies the issue, in the following pull request:

https://github.com/infinispan/infinispan/pull/1381

The Exception is:
java.lang.ClassCastException: org.infinispan.marshall.MarshalledValue cannot be cast to org.infinispan.distexec.mapreduce.Book
at org.infinispan.distexec.mapreduce.BookSearchTest$TitleBookSearcher.map(BookSearchTest.java:74)
at org.infinispan.distexec.mapreduce.MapReduceManagerImpl.map(MapReduceManagerImpl.java:179)
at org.infinispan.distexec.mapreduce.MapReduceManagerImpl.mapAndCombineForLocalReduction(MapReduceManagerImpl.java:87)
at org.infinispan.commands.read.MapCombineCommand.perform(MapCombineCommand.java:91)
at org.infinispan.remoting.InboundInvocationHandlerImpl.handleInternal(InboundInvocationHandlerImpl.java:95)
at org.infinispan.remoting.InboundInvocationHandlerImpl.handleWithWaitForBlocks(InboundInvocationHandlerImpl.java:110)
at org.infinispan.remoting.InboundInvocationHandlerImpl.handle(InboundInvocationHandlerImpl.java:82)
at org.infinispan.remoting.transport.jgroups.CommandAwareRpcDispatcher.executeCommandFromLocalCluster(CommandAwareRpcDispatcher.java:244)
at org.infinispan.remoting.transport.jgroups.CommandAwareRpcDispatcher.handle(CommandAwareRpcDispatcher.java:217)
at org.jgroups.blocks.RequestCorrelator.handleRequest(RequestCorrelator.java:483)
at org.jgroups.blocks.RequestCorrelator.receiveMessage(RequestCorrelator.java:390)
at org.jgroups.blocks.RequestCorrelator.receive(RequestCorrelator.java:248)
at org.jgroups.blocks.MessageDispatcher$ProtocolAdapter.up(MessageDispatcher.java:604)
at org.jgroups.JChannel.up(JChannel.java:715)
at org.jgroups.stack.ProtocolStack.up(ProtocolStack.java:1020)
at org.jgroups.protocols.FRAG2.up(FRAG2.java:181)
at org.jgroups.protocols.FC.up(FC.java:479)
at org.jgroups.protocols.pbcast.GMS.up(GMS.java:896)
at org.jgroups.protocols.pbcast.STABLE.up(STABLE.java:244)
at org.jgroups.protocols.UNICAST2.up(UNICAST2.java:432)
at org.jgroups.protocols.pbcast.NAKACK2.handleMessage(NAKACK2.java:722)
at org.jgroups.protocols.pbcast.NAKACK2.up(NAKACK2.java:570)
at org.jgroups.protocols.Discovery.up(Discovery.java:359)
at org.jgroups.protocols.TP.passMessageUp(TP.java:1293)
at org.jgroups.protocols.TP$IncomingPacket.handleMyMessage(TP.java:1856)
at org.jgroups.protocols.TP$IncomingPacket.run(TP.java:1829)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)