Fix recursive readPref

    When using discriminators, `readPref` is called for each schema which causes an incorrectly nested read preference (e.g., `{"_type":"ReadPreference","mode":{"_type":"ReadPreference","mode":{"_type":"ReadPreference","mode":{"_type":"ReadPreference","mode":"secondaryPreferred"}}}}`).

    This will ensure that if the `read` key is already a `ReadPref` object that we do not wrap it again.

Author: kjvalencik

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';