Merge pull request #2490 from kjvalencik/patch-1

Fix recursive `readPref`

Author: vkarpov15

lib/utils.js

@@ -472,6 +472,10 @@ exports.readPref = function readPref (pref, tags) {
     pref = pref[0];
   }
 
+  if (pref instanceof ReadPref) {
+    return pref;
+  }
+
   switch (pref) {
     case 'p':
       pref = 'primary';