adamGetReferenceString reduces pairs correctly, fixes #967

[erictu]

fixes #967

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[heuermh]

Thank you, @erictu. Could you add a unit test demonstrating the issue?

[erictu]

Done. Just to note, because the bug is nondeterministic (based on how Spark optimizes its reduce function), the unit test without the fixes doesn't always throw an error. With the new fixes, the test passes all the time.

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[heuermh]

Thanks, I understand now. LGTM

[heuermh]

Thank you, @erictu, merged as 3f21d83