goprotobuf: Use p.isUnmarshaler consistently in decode.go.
goprotobuf: Use p.isUnmarshaler consistently in decode.go.

Fixes #41.

R=crawshaw, r
CC=golang-dev
https://codereview.appspot.com/10474043

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