Set content type of digest responses to `application/vnd.opentimestam…

…ps.v1`

This is what the OTS clients say they accept. So we should give them it.

src/rpc.rs

@@ -51,7 +51,7 @@ async fn do_post_digest(
                     let stamp = stamp.serialize();
                     Ok(Response::builder()
                                 .status(StatusCode::OK)
-                                .header(http::header::CONTENT_TYPE, "application/octet-stream")
+                                .header(http::header::CONTENT_TYPE, "application/vnd.opentimestamps.v1")
                                 .body(Full::new(Bytes::from(stamp)))
                                 .unwrap())
                 },