The new android design library provides the android.support.design.widget.FloatingActionButton to make it easier to implement a floating action button.
But the button is always a circle.
I received from the company designer a square floating button concept, but I can't find any reference to change the background of the android.support.design.widget.FloatingActionButton to have a square background. 
Note: If I use a custom custom drawable the background still have the circle, like this:
<android.support.design.widget.FloatingActionButton
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@drawable/button_filter"
    app:layout_anchor="@+id/appbar"
    app:layout_anchorGravity="bottom|right|end" />

I try to use a simple Button, but it seems to break the toolbar size when it was collapsed:
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:padding="5dp"
    android:elevation="10dp"
    android:background="@drawable/button_filter"
    app:layout_anchor="@+id/appbar"
    app:layout_anchorGravity="bottom|right|end" />

Is possible to have a square button using the FloatingActionButton with the new Android Design Library?