fix(utils): restructure inheritance rules for read preferences

Author: mbroadst

lib/utils.js

@@ -517,15 +517,16 @@ function resolveReadPreference(options, sources) {
   if (options.readPreference) {
     readPreference = options.readPreference;
   } else if (session && session.inTransaction() && session.transaction.options.readPreference) {
-    // From transactions spec: If the user supplies an explicit readConcern via a method
-    // option, however, drivers MUST apply the readConcern...
+    // The transaction’s read preference MUST override all other user configurable read preferences.
     readPreference = session.transaction.options.readPreference;
-  } else if (coll && coll.s.readPreference) {
-    readPreference = coll.s.readPreference;
-  } else if (db && db.s.readPreference) {
-    readPreference = db.s.readPreference;
-  } else if (defaultReadPreference) {
-    readPreference = defaultReadPreference;
+  } else {
+    if (defaultReadPreference) {
+      readPreference = defaultReadPreference;
+    } else if (coll && coll.s.readPreference) {
+      readPreference = coll.s.readPreference;
+    } else if (db && db.s.readPreference) {
+      readPreference = db.s.readPreference;
+    }
   }
 
   // do we even have a read preference?