fix(mongo_client): translate options for connectWithUrl

Fixes NODE-1531

lib/operations/mongo_client_ops.js

@@ -296,16 +296,31 @@ function connectWithUrl(mongoClient, url, options, connectCallback) {
   // Propagate the events to the client
   relayEvents(mongoClient, url);
 
+  let finalOptions = Object.assign({}, options);
+
+  // If we have a readPreference passed in by the db options, convert it from a string
+  if (typeof options.readPreference === 'string' || typeof options.read_preference === 'string') {
+    finalOptions.readPreference = new ReadPreference(
+      options.readPreference || options.read_preference
+    );
+  }
+
   // Connect
   return url.connect(
-    options,
-    connectHandler(mongoClient, options, (err, topology) => {
+    finalOptions,
+    connectHandler(mongoClient, finalOptions, (err, topology) => {
       if (err) return connectCallback(err, topology);
-      if (options.user || options.password || options.authMechanism) {
-        return authenticate(mongoClient, options.user, options.password, options, err => {
-          if (err) return connectCallback(err, topology);
-          connectCallback(err, topology);
-        });
+      if (finalOptions.user || finalOptions.password || finalOptions.authMechanism) {
+        return authenticate(
+          mongoClient,
+          finalOptions.user,
+          finalOptions.password,
+          finalOptions,
+          err => {
+            if (err) return connectCallback(err, topology);
+            connectCallback(err, topology);
+          }
+        );
       }
 
       connectCallback(err, topology);