Merge user metadata using lodash.merge (#207)

Similar to [this example](https://auth0.com/docs/link-accounts/suggested-linking#merging-metadata), use lodash.merge to avoid loss of primary user's metadata when the secondary user has no metadata. Additionally with this change, array values are merged and primary (original) user's metadata takes precedence.

This fixes #206

src/rules/link-users-by-email-with-metadata.js

@@ -10,6 +10,7 @@
 
 function (user, context, callback) {
   const request = require('request');
+  const _ = require('lodash');
 
   // Check if email is verified, we shouldn't automatically
   // merge accounts if this is not the case.
@@ -50,11 +51,16 @@ function (user, context, callback) {
     const originalUser = data[0];
     const provider = user.identities[0].provider;
     const providerUserId = user.identities[0].user_id;
-
-    user.app_metadata = user.app_metadata || {};
-    user.user_metadata = user.user_metadata || {};
-    auth0.users.updateAppMetadata(originalUser.user_id, user.app_metadata)
-    .then(auth0.users.updateUserMetadata(originalUser.user_id, user.user_metadata))
+    const mergeCustomizer = function(objectValue, sourceValue){
+      if (_.isArray(objectValue)){
+        return sourceValue.concat(objectValue);
+      }
+    };
+    const mergedUserMetadata = _.merge({}, user.user_metadata, originalUser.user_metadata, mergeCustomizer);
+    const mergedAppMetadata = _.merge({}, user.app_metadata, originalUser.app_metadata, mergeCustomizer);
+
+    auth0.users.updateAppMetadata(originalUser.user_id, mergedAppMetadata)
+    .then(auth0.users.updateUserMetadata(originalUser.user_id, mergedUserMetadata))
     .then(function() {
       request.post({
         url: userApiUrl + '/' + originalUser.user_id + '/identities',