feat(types): support casting JSON form of buffer

re: #6863

Author: Fonger

lib/schema/buffer.js

@@ -117,9 +117,10 @@ SchemaBuffer.prototype.cast = function(value, doc, init) {
         value._subtype = this.options.subtype;
       }
     }
-
     return value;
-  } else if (value instanceof Binary) {
+  }
+
+  if (value instanceof Binary) {
     ret = new MongooseBuffer(value.value(true), [this.path, doc]);
     if (typeof value.sub_type !== 'number') {
       throw new CastError('buffer', value, this.path);
@@ -132,8 +133,12 @@ SchemaBuffer.prototype.cast = function(value, doc, init) {
     return value;
   }
 
+
   var type = typeof value;
-  if (type === 'string' || type === 'number' || Array.isArray(value)) {
+  if (
+    type === 'string' || type === 'number' || Array.isArray(value) ||
+    (type === 'object' && value.type === 'Buffer' && Array.isArray(value.data)) // gh-6863
+  ) {
     if (type === 'number') {
       value = [value];
     }

lib/types/buffer.js

@@ -44,7 +44,7 @@ function MongooseBuffer(value, encode, offset) {
   var buf;
   if (typeof val === 'number' || val instanceof Number) {
     buf = Buffer.alloc(val);
-  } else {
+  } else { // string, array or object { type: 'Buffer', data: [...] }
     buf = Buffer.from(val, encoding, offset);
   }
   utils.decorate(buf, MongooseBuffer.mixin);