Fix issue2550751: Email Header Issue. Noel Garces requested the ability to suppress email headers like "x-roundup-issue-files". With Ralf's addition of the Link/Multilink property attribute 'msg_header_property' we can do this easily. Setting the 'msg_header_property' to the empty string '' (not to None) will suppress the header for that property. 3 line code change, single test, doc updated and change note.

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