parse response body in client request bindings

[Arnaz87]

https://github.com/foundpatterns/torchbear/blob/2b7737008f925b793dff466dbdd508ece9baacf5/src/bindings/client.rs#L31-L39

For some reason rust panics when trying to parse the response body into json

[naturallymitchell]

we fixed this by using serde instead of actix-web's string JSONifier