Describe the bug
When an attribute isn't changed, the value will not be written into the serialized html comment, which in fact, makes it unaccessible for PHPs render_callback-function.

To Reproduce
Steps to reproduce the behavior:

1. Make a custom block
2. Add a sourceless attribute with default value

attributes: {
    myNumber: {
        type: "number",
        default: 1, // this number won't appear in the html comments
    }
}

3. Do not alter the attribute
4. Save the content
5. var_dump() the $attribues in 'render_callback
   5b. or look into database, the html comment doesn't contain 'number'
6. Be sad 'number' isn't there :'(

Expected behavior
To avoid syncing the defaults in both php and js, I expect gutenberg to save defaults into the html comment aswell.

Desktop (please complete the following information):

• OS: MacOS High Sierra
• Browser Doesn't matter
• Version Doesn't matter

Additional context

• Gutenberg Version 3.0.1