goprotobuf: update due to name change of reflect.ArrayCopy (now reflect.Copy)

R=nigeltao, dsymonds
CC=golang-dev
http://codereview.appspot.com/3666041
diff --git a/proto/text_parser.go b/proto/text_parser.go
index 009b3a7..cd32bf4 100644
--- a/proto/text_parser.go
+++ b/proto/text_parser.go
@@ -371,7 +371,7 @@
 		flen := fv.Len()
 		if flen == fv.Cap() {
 			nav := reflect.MakeSlice(at, flen, 2*flen+1)
-			reflect.ArrayCopy(nav, fv)
+			reflect.Copy(nav, fv)
 			fv.Set(nav)
 		}
 		fv.SetLen(flen + 1)