Setting the referer to HTTP_HOST when URI is HTTP_REFERER is not prov…

…ided by the client.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referer
issue rstudio#421

R/plumber.R

@@ -266,8 +266,12 @@ plumber <- R6Class(
           # use the HTTP_REFERER so RSC can find the swagger location to ask
           ## (can't directly ask for 127.0.0.1)
           referrer_url <- req$HTTP_REFERER
-          referrer_url <- sub("index\\.html$", "", referrer_url)
-          referrer_url <- sub("__swagger__/$", "", referrer_url)
+          if (is.null(referrer_url)) {
+            referrer_url <- paste0("http://", req$HTTP_HOST, "/")
+          } else {
+            referrer_url <- sub("index\\.html$", "", referrer_url)
+            referrer_url <- sub("__swagger__/$", "", referrer_url)
+          }
           spec$servers <- list(
             list(
               url = referrer_url,