Set updatedAt value for newly created documents

[chacha912]

What happened:

When creating a new document through the dashboard example, the updatedAt value is not displaying correctly in the document list. (Ref: yorkie-team/dashboard#99)

What you expected to happen:

When a document is first created, the updatedAt should be set to the same value as createdAt.
Reasons for this approach:

1. Initial state indication: It clearly shows that the record has not been modified since its creation.
2. Data consistency: All records will have an updatedAt value from the moment of creation.

How to reproduce it (as minimally and precisely as possible):

1. Navigate to the dashboard
2. Create a new document using the provided example
   
3. Check the document list
   
4. Observe that the updatedAt value is not correctly set

Anything else we need to know?:

Environment:

• Operating system:
• Browser and version:
• Yorkie version (use yorkie version): 0.4.30
• Yorkie JS SDK version: 0.4.28

[window9u]

Hello! I've identified an issue that needs fixing:

While working on this pull request, I realized that updated_at is only being updated when there's an operation in Change. This explains why updatedAt isn't set when a document is created.

I apologize for my mistake and will fix it immediately.

[hackerwins]

@window9u There's no need to be sorry. It seems natural that bugs are created due to changes. I'll wait for your PR.