fix(transactions): use options helper to resolve read preference

ReadPreference can be expressed a number of ways through options,
so we need to ensure that the read preference is properly resolved
when a user provides options. This change also includes fixes for
`readConcern` and `writeConcern`.

NODE-2401

Author: mbroadst

lib/core/transactions.js

@@ -1,5 +1,8 @@
 'use strict';
 const MongoError = require('./error').MongoError;
+const ReadPreference = require('./topologies/read_preference');
+const ReadConcern = require('../read_concern');
+const WriteConcern = require('../write_concern');
 
 let TxnState;
 let stateMachine;
@@ -90,18 +93,26 @@ class Transaction {
     this.state = TxnState.NO_TRANSACTION;
     this.options = {};
 
-    if (options.writeConcern || typeof options.w !== 'undefined') {
-      const w = options.writeConcern ? options.writeConcern.w : options.w;
-      if (w <= 0) {
+    const writeConcern = WriteConcern.fromOptions(options);
+    if (writeConcern) {
+      if (writeConcern.w <= 0) {
         throw new MongoError('Transactions do not support unacknowledged write concern');
       }
 
-      this.options.writeConcern = options.writeConcern ? options.writeConcern : { w: options.w };
+      this.options.writeConcern = writeConcern;
     }
 
-    if (options.readConcern) this.options.readConcern = options.readConcern;
-    if (options.readPreference) this.options.readPreference = options.readPreference;
-    if (options.maxCommitTimeMS) this.options.maxTimeMS = options.maxCommitTimeMS;
+    if (options.readConcern) {
+      this.options.readConcern = ReadConcern.fromOptions(options);
+    }
+
+    if (options.readPreference) {
+      this.options.readPreference = ReadPreference.fromOptions(options);
+    }
+
+    if (options.maxCommitTimeMS) {
+      this.options.maxTimeMS = options.maxCommitTimeMS;
+    }
 
     // TODO: This isn't technically necessary
     this._pinnedServer = undefined;