Fix catbytes (a test helper function).
Fix catbytes (a test helper function).

It was written with dependencies on the system's default character encoding,
and wound up working only on OSX.

Only tests were affected; the actual functionality worked all along.

flatland/clojure-protobuf,l0st3d/clojure-protobuf,ninjudd/clojure-protobuf,oliyh/clojure-protobuf