Change the behaviour of .query: optional params and explicit error in the response body

[LukeMathWalker]

This PR changes the behavior of .query: if the caller has not specified any query parameter, we still try to deserialise the desired type from an empty string.

This allows successful deserialisation of structs which do not have any mandatory field, e.g.

struct OptionalParams {
   author: Option<String>,
   theme: Option<String>
}

On master, this requires using unwrap_or on the outcome of .query which I think it's unnecessary.
If you forget about it, it leads to surprising behavior: calling the endpoint with any query parameter (none of which in the struct) would trigger a successful deserialisation, while a call without query params would result in a 400.

I have also added, for the unhappy case, the deserialisation error in the response body: it makes for a much more pleasant experience when trying to debug why an endpoint is returning Bad Request 😅

[yoshuawuyts]

This is great; thank you!