Requests should have `Content-Type: application/json`

[quad]

The documentation strongly suggests that the content-type is set to JSON, but this doesn't happen anywhere!

We're using the following middleware … but it would be nice if it was included by default.

pub struct ContentTypeMiddleware;

impl MiddleWare for ContentTypeMiddleware {
    fn request<E>(
        &self,
        _: &E,
        req: &mut http::request::Request<Vec<u8>>,
    ) -> Result<(), ClientError>
    where
        E: Endpoint,
    {
        req.headers_mut()
            .append("content-type", HeaderValue::from_static("application/json"));

        Ok(())
    }

    fn response<E>(
        &self,
        _: &E,
        _: &mut http::response::Response<Vec<u8>>,
    ) -> Result<(), ClientError>
    where
        E: Endpoint,
    {
        Ok(())
    }
}

Would you be open to a PR adding this default behaviour?