fix(collection): depend on resolveReadPreference for inheritance

We no longer need to explicitly copy `readPreference` from the
parent `Collection` here, because we already handle inheritance
in `resolveReadPreference`.

Author: mbroadst

lib/collection.js

@@ -84,7 +84,7 @@ const updateOne = require('./operations/collection_ops').updateOne;
  * });
  */
 
-const mergeKeys = ['readPreference', 'ignoreUndefined'];
+const mergeKeys = ['ignoreUndefined'];
 
 /**
  * Create a new Collection instance (INTERNAL TYPE, do not instantiate directly)
@@ -322,7 +322,8 @@ Collection.prototype.find = function(query, options, callback) {
       : { _id: 1 };
   }
 
-  let newOptions = {};
+  // Make a shallow copy of options
+  let newOptions = Object.assign({}, options);
 
   // Make a shallow copy of the collection options
   for (let key in this.s.options) {
@@ -331,11 +332,6 @@ Collection.prototype.find = function(query, options, callback) {
     }
   }
 
-  // Make a shallow copy of options
-  for (let optKey in options) {
-    newOptions[optKey] = options[optKey];
-  }
-
   // Unpack options
   newOptions.skip = options.skip ? options.skip : 0;
   newOptions.limit = options.limit ? options.limit : 0;