NIFI-4950 Defining behavior for MergeContent when more than 1 FlowFil…

[markobean]

…e has the same fragment.index value

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[mosermw]

Reviewed and tested; behavior in the presence of a duplicate fragment is fixed; duplicate is sent to failure. I fixed a typo and used FragmentAttributes.FRAGMENT_INDEX enum in place of "fragment.index" strings.
+1, merging. Thanks for fixing this @markobean.