Wrap responses in a 'value' key.

The approach here is a little hacky but it allows us to continue using
rustc_serialize to encode the data and avoids changing the internal API.

src/response.rs

@@ -20,18 +20,29 @@ pub enum WebDriverResponse {

impl WebDriverResponse {
    pub fn to_json_string(self) -> String {
        match self {
            WebDriverResponse::CloseWindow(x) => json::encode(&x),
            WebDriverResponse::Cookie(x) => json::encode(&x),
        let obj = match self {
            WebDriverResponse::CloseWindow(ref x) => json::encode(x),
            WebDriverResponse::Cookie(ref x) => json::encode(x),
            WebDriverResponse::DeleteSession => Ok("{}".to_string()),
            WebDriverResponse::ElementRect(x) => json::encode(&x),
            WebDriverResponse::Generic(x) => json::encode(&x),
            WebDriverResponse::NewSession(x) => json::encode(&x),
            WebDriverResponse::Timeouts(x) => json::encode(&x),
            WebDriverResponse::ElementRect(ref x) => json::encode(x),
            WebDriverResponse::Generic(ref x) => json::encode(x),
            WebDriverResponse::NewSession(ref x) => json::encode(x),
            WebDriverResponse::Timeouts(ref x) => json::encode(x),
            WebDriverResponse::Void => Ok("{}".to_string()),
            WebDriverResponse::WindowPosition(x) => json::encode(&x),
            WebDriverResponse::WindowSize(x) => json::encode(&x),
        }.unwrap()
            WebDriverResponse::WindowPosition(ref x) => json::encode(x),
            WebDriverResponse::WindowSize(ref x) => json::encode(x),
        }.unwrap();

        match self {
            WebDriverResponse::Generic(_) => obj,
            _ => {
                let mut data = String::with_capacity(11 + obj.len());
                data.push_str("{\"value\": ");
                data.push_str(&*obj);
                data.push_str("}");
                data
            }
        }
    }
}