goprotobuf: Explicitly type the generated enum constants.

R=r
CC=golang-dev
http://codereview.appspot.com/4952045
diff --git a/proto/all_test.go b/proto/all_test.go
index dd7f66e..256ab4c 100644
--- a/proto/all_test.go
+++ b/proto/all_test.go
@@ -112,7 +112,7 @@
 		pb.F_Sint64Defaulted = Int64(Default_GoTest_F_Sint64Defaulted)
 	}
 
-	pb.Kind = Int32(GoTest_TIME)
+	pb.Kind = NewGoTest_KIND(GoTest_TIME)
 	pb.RequiredField = initGoTestField()
 	pb.F_BoolRequired = Bool(true)
 	pb.F_Int32Required = Int32(3)