goprotobuf: hand-edit the gofix changes for nicer code
and clean up a few details.  No semantic change.

R=rsc, dsymonds
CC=golang-dev
http://codereview.appspot.com/4368069
diff --git a/proto/encode.go b/proto/encode.go
index 62674d1..8dc6a74 100644
--- a/proto/encode.go
+++ b/proto/encode.go
@@ -292,7 +292,7 @@
 // All protocol buffer fields are nillable, but be careful.
 func isNil(v reflect.Value) bool {
 	switch v.Kind() {
-	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
+	case reflect.Map, reflect.Ptr, reflect.Slice:
 		return v.IsNil()
 	}
 	return false