Group column header is not styled correctly when moving that column

[SAndreeva]

Description

Group column header is not styled correctly when moving that column

• igniteui-angular version: 6.2.x
• browser: N/A

Steps to reproduce

1. Switch to SAndreeva/header-group-component-6.2.x
2. Run Grid Column Groups demo
3. Start dragging the 'General Information' column group.
4. Observe the style applied on the header group.

Result

Not all of the titles receive the same background color, etc.

Expected result

All of the titles receive the same background color, etc.

[simeonoff]

@SAndreeva Can you elaborate a bit more?