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

Fixes NODE-1409

* fix(serializer): map insert expects only string keys

Checks that the key is a string before checking that
the key is a valid string to prevent type error.

Fixes NODE-1572

Author: kvwalker

lib/bson/parser/serializer.js

@@ -15,6 +15,7 @@ var normalizedFunctionString = require('./utils').normalizedFunctionString;
 // }
 
 var regexp = /\x00/; // eslint-disable-line no-control-regex
+var ignoreKeys = ['$db', '$ref', '$id', '$clusterTime'];
 
 // To ensure that 0.4 of node works correctly
 var isDate = function isDate(d) {
@@ -798,7 +799,7 @@ var serializeInto = function serializeInto(
       type = typeof value;
 
       // Check the key and throw error if it's illegal
-      if (key !== '$db' && key !== '$ref' && key !== '$id') {
+      if (typeof key === 'string' && ignoreKeys.indexOf(key) === -1) {
         if (key.match(regexp) != null) {
           // The BSON spec doesn't allow keys with null bytes because keys are
           // null-terminated.
@@ -899,7 +900,7 @@ var serializeInto = function serializeInto(
       type = typeof value;
 
       // Check the key and throw error if it's illegal
-      if (key !== '$db' && key !== '$ref' && key !== '$id') {
+      if (typeof key === 'string' && ignoreKeys.indexOf(key) === -1) {
         if (key.match(regexp) != null) {
           // The BSON spec doesn't allow keys with null bytes because keys are
           // null-terminated.