feat(buffer): add support for subtype prop

Fix #5530

lib/schema/buffer.js

@@ -113,6 +113,9 @@ SchemaBuffer.prototype.cast = function(value, doc, init) {
   if (Buffer.isBuffer(value)) {
     if (!value || !value.isMongooseBuffer) {
       value = new MongooseBuffer(value, [this.path, doc]);
+      if (this.options.subtype != null) {
+        value._subtype = this.options.subtype;
+      }
     }
 
     return value;
@@ -135,12 +138,36 @@ SchemaBuffer.prototype.cast = function(value, doc, init) {
       value = [value];
     }
     ret = new MongooseBuffer(value, [this.path, doc]);
+    if (this.options.subtype != null) {
+      ret._subtype = this.options.subtype;
+    }
     return ret;
   }
 
   throw new CastError('buffer', value, this.path);
 };
 
+/**
+ * Sets the default [subtype](https://studio3t.com/whats-new/best-practices-uuid-mongodb/)
+ * for this buffer. You can find a [list of allowed subtypes here](http://api.mongodb.com/python/current/api/bson/binary.html).
+ *
+ * ####Example:
+ *
+ *     var s = new Schema({ uuid: { type: Buffer, subtype: 4 });
+ *     var M = db.model('M', s);
+ *     var m = new M({ uuid: 'test string' });
+ *     m.uuid._subtype; // 4
+ *
+ * @param {Number} subtype the default subtype
+ * @return {SchemaType} this
+ * @api public
+ */
+
+SchemaBuffer.prototype.subtype = function(subtype) {
+  this.options.subtype = subtype;
+  return this;
+};
+
 /*!
  * ignore
  */