do not omit Integer, Float and Character valued fields from RadioMessage

not complete yet, needs additional tests for release

issue #579

(cherry picked from commit d30c6a00e351d6c0ce423cd6dadb1478f3ad7cf4)
add test for RadioMessage.serialize

fixes #579

(cherry picked from commit 8a1c5c0a670987bebfe5bae6e6cdea42e40942b0)