response$send_file() and root = "/" fix on Windows

We cannot just use `file.path()` because if prepends the
`/` in front of the device letter: `/c:/foo...`.

NEWS.md

@@ -1,5 +1,8 @@
 # webfakes (development version)
 
+* `response$send_file()` now handles `root = "/"` and absolute paths
+  better on Windows.
+
 * `new_app_process()` and `local_app_process()` are now faster,
   because the app object they need to copy to the subprocess is smaller.
 

R/response.R

@@ -199,7 +199,14 @@ new_response <- function(app, req) {
         }
       }
 
-      self$send(read_bin(normalizePath(file.path(root, path))))
+      if (root == "/" && .Platform$OS.type == "windows" &&
+          grepl("^[a-zA-Z]:", path)) {
+        abs_path <- path
+      } else {
+        abs_path <- file.path(root, path)
+      }
+
+      self$send(read_bin(normalizePath(abs_path)))
     },
 
     send_status = function(status) {