refactor: resolveReadPreference always returns a ReadPreference

We use this method to optionally accept operation-level read
preference overrides, but we were just accepting those methods
passing in a string. This lets us ensure that we are _always_
dealing with a proper `ReadPreference` instance.

lib/utils.js

@@ -519,7 +519,7 @@ function resolveReadPreference(parent, options) {
     throw new Error('No readPreference was provided or inherited.');
   }
 
-  return readPreference;
+  return typeof readPreference === 'string' ? new ReadPreference(readPreference) : readPreference;
 }
 
 /**