NIFI-10203: Fixed bug in which same FlowFile Builder was used repeate…

[markap14]

…dly for multiple FlowFiles; this caused mingling of their attributes when a FlowFile is routed to a relationship that has more than 1 destination (i.e., many connections with the same Relationship)

Summary

NIFI-00000

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[greyp9]

This change looks good. Thanks for the unit test, it illustrated the problem exactly.

I triggered the MacOS automation again; I expect that the failure was a transient.