goprotobuf: Make UnmarshalText reset the message before unmarshaling.
goprotobuf: Make UnmarshalText reset the message before unmarshaling.

This is the expected behaviour, and matches Unmarshal.

R=r
CC=golang-dev
https://codereview.appspot.com/12449043

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