Allow Iron URLs to convert to rust-url URLs.

I've added a method to convert our custom URL type to a `rust-url::Url`.

It takes `self` by value rather than reference, as inner fields need cloning anyway. This way we can do both:

```rust
// Original url no longer valid.
let rust_url = url.into_generic_url();

// Original url still valid.
let rust_url = url.clone().into_generic_url();
```

Relevant: https://github.com/iron/iron/pull/197