Bug: matSortDirection will not work if matSortActive had changed in matSort directive.

#### Bug, feature request, or proposal:
**Bug!**

#### What is the expected behavior?
In the table element with sort-header - matSort directive - the properties **matSortActive** and **matSortDirection** will work independently.

#### What is the current behavior?
The **matSortDirection** property will work only if **matSortActive** didn't changed

#### What are the steps to reproduce?
1. Create a new Component. In it's template (html file) create a table element with **matSort** directive.
2. Add to this table the properties [matSortActive] and [matSortDirection] and bind them to variables in your Component
3. In the Component initialization (constructor or ngInit) set values for the variables binded to matSortActive and matSortDirection.
4. Add a method_1 that will update the matSortDirection only - **Verify:** The Sort-Arrow in the table's header appear in the right direction (asc/desc).
5. Add a method_2 that will update the matSortDirection **and** matSortActive - **Bug:** The Sort-Arrow in the table's header is missing!!!

Providing a StackBlitz reproduction is the *best* way to share your issue.

StackBlitz starter:

View and Edit: https://stackblitz.com/edit/angular-41eoff?file=main.ts

View Only: https://angular-41eoff.stackblitz.io

#### What is the use-case or motivation for changing an existing behavior?
Fixing the bug and providing a current behavior for these 2 properties (matSortActive and matSortDirection).

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
- Angular v6.1.3
- Material v6.4.5
- TypeScript v2.9.2
- Browsers: Tested in Chrome, Opera (maybe others).

Beginne mit der verknüpften StackBlitz-Reproduktion und der Angular Material matSort-Direktive und reproduziere dann den Fall, in dem sich matSortActive und matSortDirection gemeinsam ändern. Die Fehlerbehebung ist abgeschlossen, wenn die Änderung beider gebundener Eigenschaften den Sortierpfeil in der angeforderten Richtung sichtbar hält und ein Regressionstest dieses Update abdeckt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.