Fixed header value to lower case in header-name for slot

[devkuma]

I am using your code, thank you for making good code.
I find a bug in this project and request a fix.

Type

What types of changes does your code introduce?

• #header-{name} - If the name contains uppercase letters, it is not applied.
  • For example, createTime and userType do not apply.
• https://hc200ok.github.io/vue3-easy-data-table-doc/features/header-slot.html

Put an x in the boxes that apply_

• Fix
• Feature

Checklist

Put an x in the boxes that apply._

• Title as described
• Add unit test by vitest if necessary