How to set a header in custom DataFormat?

[Chris]

I implemented a custom DataFormat and in the "unmarshal(Exchange 
exchange, InputStream stream)" implementation I set a header, but upon 
attempting to retrieve the header downstream from the "unmarshal" call,
the header is not there.  Since the pattern is inOnly, I added the 
header to the in Message.  I've done similar with custom Processors and 
in that case, it works.  What is different with DataFormat?  Is there 
any way to set header(s) from DataFormat marshal/unmarshal?

Thanks,
Chris