fix(serializer): do not use checkKeys for $clusterTime

Fixes NODE-1409

Author: kvwalker

Diff for: lib/parser/serializer.js

@@ -10,6 +10,7 @@ const constants = require('../constants');
 const normalizedFunctionString = require('./utils').normalizedFunctionString;
 
 const regexp = /\x00/; // eslint-disable-line no-control-regex
+const ignoreKeys = new Set(['$db', '$ref', '$id', '$clusterTime']);
 
 // To ensure that 0.4 of node works correctly
 const isDate = function isDate(d) {
@@ -785,7 +786,7 @@ function serializeInto(
       const type = typeof value;
 
       // Check the key and throw error if it's illegal
-      if (key !== '$db' && key !== '$ref' && key !== '$id') {
+      if (!ignoreKeys.has(key)) {
         if (key.match(regexp) != null) {
           // The BSON spec doesn't allow keys with null bytes because keys are
           // null-terminated.
@@ -885,7 +886,7 @@ function serializeInto(
       const type = typeof value;
 
       // Check the key and throw error if it's illegal
-      if (key !== '$db' && key !== '$ref' && key !== '$id') {
+      if (!ignoreKeys.has(key)) {
         if (key.match(regexp) != null) {
           // The BSON spec doesn't allow keys with null bytes because keys are
           // null-terminated.