Support deserializing optional fields

```rust
#[macro_use]
extern crate serde_derive;

extern crate serde_urlencoded;

#[derive(Deserialize, Serialize, Debug)]
struct S {
    s: Option<String>,
}

fn main() {
    println!("{:?}", serde_urlencoded::from_str::<S>("s=abc"));
}
```