Usage of header slot produces JavaScript error when beeing combined with ColumnGroups

[AlexanderGrasberger]

Vue produces a error when using <template #header> inside of a ColumnGroup like so:

<ColumnGroup type="header">
    <Row>
        <Column field="someField" :sortable="true">
            <template #header>
               Custom header
            </template>
        </Column>
    </Row>
</ColumnGroup>

The issue is being caused by the usage of a component ColumnSlot within the TableHeader.vue.
In the registration for the component the name was changed to DTColumnSlot.

[AlexanderGrasberger]

I have sent a pull request according to this problem: #345

[cagataycivici]

Thank you sir!