Support webdav request method

[abersheeran]

I have reviewed the Rust source code, and currently it is using an enumeration type. However, if it is changed to a String, then it will be able to support any HTTP request method just like in a JavaScript environment.

[frugality4121]

https://docs.rs/http/latest/http/method/struct.Method.html

How about using http::Method?

use http::Method as HttpMethod;
use worker::{Method as WorkerMethod, *};

fn handle_request(req: Request, env: Env) -> Result<Response> {
    let worker_method: WorkerMethod = req.method();
    let method_str = worker_method.as_str();

    let http_method = HttpMethod::from_bytes(method_str.as_bytes())
        .unwrap_or(HttpMethod::GET);

    // ...

    Response::ok("Hello!")
}