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).

Comienza con la reproducción vinculada de StackBlitz y la directiva matSort de Angular Material; después, reproduce el caso en el que matSortActive y matSortDirection cambian conjuntamente. La corrección estará completa cuando cambiar ambas propiedades enlazadas mantenga visible la flecha de ordenación en la dirección solicitada, con una prueba de regresión que cubra esa actualización.

Escrito por el modelo de indexación a partir del texto del issue.