fix: ReadPreference maxStalenessSeconds from options

NODE-2678

Author: Thomas Reggi

lib/core/topologies/read_preference.js

@@ -91,20 +91,18 @@ const VALID_MODES = [
  * @return {ReadPreference}
  */
 ReadPreference.fromOptions = function(options) {
+  if (!options) return null;
   const readPreference = options.readPreference;
+  if (readPreference == null) return null;
   const readPreferenceTags = options.readPreferenceTags;
-
-  if (readPreference == null) {
-    return null;
-  }
-
+  const maxStalenessSeconds = options.maxStalenessSeconds;
   if (typeof readPreference === 'string') {
     return new ReadPreference(readPreference, readPreferenceTags);
   } else if (!(readPreference instanceof ReadPreference) && typeof readPreference === 'object') {
     const mode = readPreference.mode || readPreference.preference;
     if (mode && typeof mode === 'string') {
       return new ReadPreference(mode, readPreference.tags, {
-        maxStalenessSeconds: readPreference.maxStalenessSeconds
+        maxStalenessSeconds: readPreference.maxStalenessSeconds || maxStalenessSeconds
       });
     }
   }