fix: diverging axum route and openapi spec

[boemjay]

Currently there is the possibility that the swagger-ui does not work with utoipa_axum in certain conditions.
The reason for this is, that the path resolution in the .nest(router) function is different in axum and utoipa.

This pull request fixes it by applying the same logic for the OpenAPI spec generation, that is used in axum.

How to reproduce this issue

With the following minimal reproducible example, open the swagger ui on localhost:3000/docs.
The only endpoint is /a/, but when you perform the request, it returns 404 Not Found.

use std::io;
use utoipa::OpenApi;
use utoipa_axum::{router::OpenApiRouter, routes};
use utoipa_swagger_ui::SwaggerUi;

#[derive(OpenApi)]
#[openapi()]
struct ApiDoc;

#[tokio::main]
async fn main() -> Result<(), io::Error> {
    #[utoipa::path(get, path = "/")]
    async fn nested_get_all() -> String {
        String::from("nested get all")
    }

    let (router, api) = OpenApiRouter::with_openapi(ApiDoc::openapi())
        .nest("/a", OpenApiRouter::new().routes(routes!(nested_get_all)))
        .split_for_parts();

    let router = router.merge(SwaggerUi::new("/docs").url("/apidoc/openapi.json", api));

    let listener = tokio::net::TcpListener::bind(("0.0.0.0", 3000)).await?;
    axum::serve(listener, router).await
}

Cause of the issue

In Axum the .nest logic joins the prefix and nested path in a different way than utopia. E.g. for nested routes the trailing / is stripped away and other logic to prevent double // in the joined path.

[juhaku]

This is pretty neat, 👍

However could you add a line to CHANGELOG.md for the addition at beginning of file.

## Unreleased

### Fixed

* title (http;//PR)

[juhaku]

I added the CHANGELOG markings for the PR 👌