http: a way to send a write that must not be applied twice

A POST that failed in transport -- including a timeout while waiting for the
answer -- is resent today, and so is one that got a 502 or 504. For a write
such as Email/import the server may already have applied the first copy,
so the resend can create a duplicate.

post_json_once and Request::send_once send it once: a transport failure or
a gateway error comes back to the caller, which can check the target before
trying again. 429 and 503, which mean the request was not processed, are
still retried.

src/jmap/request.rs

@@ -131,6 +131,14 @@ impl Request {
         let value = client.post_json(api_url, &self.envelope()?)?;
         Response::parse(value)
     }
+
+    /// As `send`, for a write that must not be applied twice: a transport
+    /// failure comes back as an error instead of being resent (see
+    /// `HttpClient::post_json_once`).
+    pub fn send_once(&self, client: &HttpClient, api_url: &str) -> Result<Response, JmapError> {
+        let value = client.post_json_once(api_url, &self.envelope()?)?;
+        Response::parse(value)
+    }
 }
 
 #[derive(Debug)]