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: Sophie Saskin

lib/parser/serializer.js

@@ -786,7 +786,7 @@ function serializeInto(
       const type = typeof value;
 
       // Check the key and throw error if it's illegal
-      if (!ignoreKeys.has(key)) {
+      if (typeof key === 'string' && !ignoreKeys.has(key)) {
         if (key.match(regexp) != null) {
           // The BSON spec doesn't allow keys with null bytes because keys are
           // null-terminated.
@@ -886,7 +886,7 @@ function serializeInto(
       const type = typeof value;
 
       // Check the key and throw error if it's illegal
-      if (!ignoreKeys.has(key)) {
+      if (typeof key === 'string' && !ignoreKeys.has(key)) {
         if (key.match(regexp) != null) {
           // The BSON spec doesn't allow keys with null bytes because keys are
           // null-terminated.