riemann: Always exclude attributes that conflict with properties

Since attributes get merged into the same dictionary as properties on
the Riemann side, do not allow - accidentally - sending attributes with
the same name. To this end, implicitly exclude these headers. Many
thanks to Fabien Wernli for the report and the solution.

This fixes #260.

Reported-by: Fabien Wernli
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>