Update documents with fields with strings array leads to changes type: SS->LIST

[komanton]

Describe the bug
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To Reproduce
Steps to reproduce the behavior:

1. View document
   { "username": "test@mail.com", "grps": [ "group-id-1", "group-id-2" ] }
2. Press Save button
3. Get document with dynamoose where scheme looks like this:

const schema = new Schema({
  username: {
    type: String,
    hashKey: true
  },
 grps: {
    type: [String],
    default: []
  }
});

export default dynamoose.model<SomeInterfaceSchema, unknown>("Name", schema );

Dynamoose will fail because type of "grps" is changed from SS to LIST

Expected: step 2 should not change the type of "grps" to LIST

Expected behavior
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Screenshots
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Desktop (please complete the following information):

• OS: Windows
• Browser chrome
• Version [e.g. 22]

Additional context
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