Reset the state of matrix object using a proper reset method.

This library is pretty awesome, I'm really happy with that! :)

Currently I'm having an issue when trying to run Robolectric tests that inflates layouts containing a RoundedImageView:

```
Caused by: java.lang.NullPointerException: can't get a shadow for null
	at org.robolectric.bytecode.ShadowWrangler.shadowOf(ShadowWrangler.java:432)
	at org.robolectric.Robolectric.shadowOf_(Robolectric.java:1034)
	at org.robolectric.Robolectric.shadowOf(Robolectric.java:741)
	at org.robolectric.shadows.ShadowMatrix.set(ShadowMatrix.java:64)
	at android.graphics.Matrix.set(Matrix.java)
```

Since android.graphics.Matrix.set(param) does a null check, and calls android.graphics.Matrix.reset() when the param is null - [see android.graphics.Matrix code, line 260](http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/4.4.4_r1/android/graphics/Matrix.java?av=f), it might not be a problem to directly call the reset instead of set(null). This would keep everything working as before, and would make Robolectric happy! =]