fix deserializing Option::Some

[jobafr]

Hi,

there's a bug when Deserialize-ing a struct that contains an Option as one of its fields. We use this via rosrust to query a substructure of interrelated configuration parameters in a typed manner.

Example to reproduce:

use serde::Deserialize;

#[derive(Debug, Deserialize)]
struct Config {
    a : String,
    b : Option<i64>,
    sub : Option<SubStruct>
}

#[derive(Debug, Deserialize)]
struct SubStruct {
    c : Option<f32>
}

fn main() {
    rosrust::init("node");
    let config = rosrust::param("/config").unwrap().get::<Config>().unwrap();
    println!("{:#?}", config);
}

Load this YAML file as rosparams (rosparam load params.yaml):

config:
  a: "hello"
  sub:
    c: 3.1

Then cargo run it. With the current git main branch of xml-rpc-rs, we get

thread 'main' panicked at src/main.rs:17:69:
called `Result::unwrap()` on an `Err` value: Server("Response data has unexpected structure: Issue while decoding data structure: invalid value: map, expected option")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

With this patch, we get:

Config {
    a: "hello",
    b: None,
    sub: Some(
        SubStruct {
            c: Some(
                3.1,
            ),
        },
    ),
}

Note that Option now behaves as expected for the Some case as well as the None case (the latter did work already before).