Add header Enrich to inbound sftp [INT-2949]

[spring-operator]

Y A opened INT-2949 and commented

There is not currently a way to provide a header mapper (to generate custom headers) on the (s)ftp endpoints.
it could be handy to enrich inbound sftp with something like:
<int:header name="app.PartnerId" ref="MySftpFactory" method="getPartner" />

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Affects: 2.2.1

Reference URL: http://forum.springsource.org/showthread.php?135279-dynamic-inbound-sftp-and-double-messages-into-the-queue