fix(dgw): Content-Type header present twice for Json responses (#315)

This is bad.

Indeed, `Content-Type` is a "singleton field": a single member is anticipated as the field value.

RFC9110 says:

> Although Content-Type is defined as a singleton field,
> it is sometimes incorrectly generated multiple times,
> resulting in a combined field value that appears to be a list.
> Recipients often attempt to handle this error by using
> the last syntactically valid member of the list, leading to
> potential interoperability and security issues if different
> implementations have different error handling behaviors.

This was a bug in Saphir. I fixed it and updated the reference in Cargo.lock.