Allow to update doc with a discriminatorKey in $set

[abarriel]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

Hello, I would like to knows, if we can allow updating a doc with a data containing the discriminatorKey, but the value of the discriminatorKey is the same, thus no override

Motivation

This allows user to perform a full update, without having to taking care of removing discriminatorKey from is updatedData

Example

diff --git a/lib/helpers/query/castUpdate.js b/lib/helpers/query/castUpdate.js
index 2e1dde1a6c0827f65e2be252cf317e329c927566..cd9f58f6aca18e3eb44ede56794e478bd3de43d8 100644
--- a/lib/helpers/query/castUpdate.js
+++ b/lib/helpers/query/castUpdate.js
@@ -207,7 +207,7 @@ function walkUpdatePath(schema, obj, op, options, context, filter, pref) {
     if (
       schema.discriminatorMapping != null &&
       discriminatorKey === schema.options.discriminatorKey &&
-      !options.overwriteDiscriminatorKey
+      !options.overwriteDiscriminatorKey && schema.discriminatorMapping.value !== obj[key]
     ) {
       if (strictMode === 'throw') {
         const err = new Error('Can\'t modify discriminator key "' + discriminatorKey + '" on discriminator model');