Make cargo compile against the nightly (url crate changes)

On rust-lang/rust@ed47c479d73fd7b57d3b493e03c74e2932733163 (which landed yesterday via rust-lang/rust@b8ef5cf1310a7b1e31d0993885d867a6804597ad), `url.path` now returns a `Path`, not a `String`. This one-line patch makes cargo compile again (at least as of rust-lang/rust@b8ef5cf1310a7b1e31d0993885d867a6804597ad, the `crate_id` changes haven't reached me yet).