fix(datepicker): change overlay position strategy so the calendar is kept on-screen (#11607)
fix(datepicker): change overlay position strategy so the calendar is kept on-screen (#11607)

* change withPush strategy to true

* address @crisbeto's comment

josephperrott/material2,mmalerba/material2,angular/components,crisbeto/material2,josephperrott/material2,jelbourn/material2,andrewseguin/material2,Jefiozie/material2,andrewseguin/material2,crisbeto/material2,andrewseguin/material2,mmalerba/material2,angular/components,jelbourn/material2,jelbourn/material2,josephperrott/material2,angular/components,josephperrott/material2,josephperrott/material2,crisbeto/material2,DevVersion/material2,Jefiozie/material2,mmalerba/material2,andrewseguin/material2,Jefiozie/material2,Jefiozie/material2,andrewseguin/material2,mmalerba/material2,DevVersion/material2,jelbourn/material2,DevVersion/material2,DevVersion/material2,angular/components,jelbourn/material2,crisbeto/material2,mmalerba/material2,Jefiozie/material2,DevVersion/material2