This removes a small foot gun from the routing.
This means matching different HTTP methods for the same route that
aren't defined together now works.
So `Router::new().route("/", get(...)).route("/", post(...))` now
accepts both `GET` and `POST`. Previously only `POST` would be accepted.
This way there is now only one way to create a router:
```rust
use axum::{Router, handler::get};
let app = Router::new()
.route("/foo", get(handler))
.route("/foo", get(handler));
```
`nest` was changed in the same way:
```rust
use axum::Router;
let app = Router::new().nest("/foo", service);
```
It was removed as part of https://github.com/tokio-rs/axum/pull/184 but
I do actually think it has some utility. So makes sense to keep even if
axum doesn't use it directly for routing.
David Pedersen