fix: propagate sortDir changes to template

[ebiermann]

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
The property sortDir of header DataTableHeaderCellComponent is not updated in the template after a sort is triggered. See last comments in #808

What is the new behavior?
sortDir is propagated to cellcontext.sortdir and therefore updated when accessed via ngTemplateOutletContext inside templates

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[ebiermann]

I also wanted to add a regression test to header-cell.component.spec.ts but was not able to prepare the component properly:

it('should` propagate sortDir changes to template', () => {
      component.column = {
        sortable: true
      };
      // check if initial sortDir matches template(ngTemplateOutletContext) sortDir
      const previousSortDir = component.sortDir;
      expect(component.cellContext.sortDir).toBe(component.sortDir);

      component.onSort();

      // check if sortDir changed and new sortDir matches template(ngTemplateOutletContext) sortDir
      expect(component.sortDir).not.toBe(previousSortDir);
      expect(component.cellContext.sortDir).toBe(component.sortDir);
    });

[amcdnl]

LGTM

[ahsanahmad22]

Hi @ebiermann

I am using external column sort but I want to sort column data on first load of page, and show the correct icon for that. I have overridden the onSort(event); How I can use this method from ngInit().