no way to determine if an optional field is actually set or unset.

```
as far as i can tell there is no way to determine whether a field marked 
"optional" has in fact been 
set or not. ie, i don't see a method corresponding to the 
message.has_my_field() functionality on 
the C++ side.

is this so ?
any thoughts on how to add such ?

```

Original issue reported on code.google.com by `DocOce...@gmail.com` on 5 Apr 2010 at 5:36
