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?

まず、月と年だけを設定した構成を使用して、showroom URL の Angular Material datepicker month picker の動作を再現します。ヘッダーのクリック動作を追跡し、完了した変更によって日付選択に切り替わらず、月と年の選択画面に留まることを確認します。

索引モデルが issue の本文から書いたものです。