goprotobuf: Add tests for the Marshaler interface during marshaling.
goprotobuf: Add tests for the Marshaler interface during marshaling.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/87130044

aoeu/protobuf,qinlodestar/protobuf,skyportsystems/go-protobuf,csuideal/protobuf,jrbarron/protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,golang/protobuf,ekle/protoc-gen-goweb_test,golang/protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,trdata/protobuf,alskjstl/protobuf,daemonl/protobuf,tamird/protobuf-2,hailocab/protobuf,linkedin-inc/protobuf,skyportsystems/go-protobuf,jmptrader/protobuf-1,ArtemTeleshev/go-protobuf,francoishill/protobuf,trdata/protobuf,mwitkow-io/protobuf,tamird/protobuf-2,kung-foo/protobuf,velocity-toolkit/protobuf-1,jeremyletang/protobuf,adnaan/protobuf,eduardiazf/protobuf,ekle/protoc-gen-goweb