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

[komanton]

Describe the bug A clear and concise description of what the bug is.

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**
A clear and concise description of what you expected to happen.

**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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