Auto merge of #2737 - aidanhs:aphs-hash-url-serialization, r=alexcrichton

Hash url serialization to ensure hash stability

`as_str` is documented to return the serialization of a url (http://servo.github.io/rust-url/url/struct.Url.html#method.as_str).
The serialization of a url has a spec (https://url.spec.whatwg.org/#url-serializing).

Hashing the serialization of a url insulates cargo against possible decisions to alter how rust-url does hashing (I've observed it happening twice). Note that rust-url happens to do this internally (but is not guaranteed to in future) and so this change doesn't actually have any effect on hash values.

r? @alexcrichton

Closes #1710