Monthpicker switch to days calendar on header click

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

Bug
#### What is the expected behavior?

When you configure datepicker with only month and year, the click on calendar header must switch between month and years chooser

#### What is the current behavior?
When you configure datepicker with only month and year, the click on calendar header switch on days chooser

#### What are the steps to reproduce?
On the showroom : https://material.angular.io/components/datepicker/overview
go to the month picker, open calendar and click on year in the header

#### What is the use-case or motivation for changing an existing behavior?
To have a real month picker

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
6+, 7+

#### Is there anything else we should know?

Start by reproducing the behavior in the Angular Material datepicker month picker at the showroom URL, using a configuration with only month and year. Trace the header click behavior and verify that the completed change keeps the chooser on month and year selection instead of switching to the days chooser.

Written by the indexing model from the issue text.