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

t3rm1n4l/goprotobuf,polydawn-ports/goprotobuf,tonnerre/go-protobuf