25: Deserialize () from input without key/value pairs. r=nox
Currently there's no input that can be deserialized to `()`, this changes the crate to support deserializing strings without any key/value pair to a `()` (such as the empty string, or just `"&"`).

Would be great if you could release a patch for this, I have a project that needs this behavior.

EDIT: To expand on my use case, I have a trait which parses the body of an HTTP Request into a type (e.g. for a `POST` request). However, this is abstract over HTTP methods, and its expected to be able to parse an empty body (e.g. from a `GET` request) into `()`.