This commit introduces support for method-less routes and route
attributes, which match _any_ valid method: `#[route("/")]`. The `Route`
structure's `method` field is now accordingly of type `Option<Route>`.
The syntax for the `route` attribute has changed in a breaking manner.
To set a method, a key/value of `method = NAME` must be introduced:
```rust
#[route("/", method = GET)]
```
If the method's name is a valid identifier, it can be used without
quotes. Otherwise it must be quoted:
```rust
// `GET` is a valid identifier, but `VERSION-CONTROL` is not
#[route("/", method = "VERSION-CONTROL")]
```
Closes#2731.
72c91958b7