Fix mock object to match changes from int to double for channel

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Check that https://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-integration-java/lastCompletedBuild/testngreports/integration/integration.ImporterTest/testImportImageWithAcquisitionData/? is green
I would think this is just an effort to have something more realistic. If the tests are passing, then it should be fine.