Expected behavior

Calling show() on ToastService should support a message and a toast type, like so: show('This is an error message', null, null, 'error'). This worked in the 8.0.0 release

Actual behavior

There are a few issues here:

1. There isn't a defensive check on the override configuration, so specifying null causes an error in the ToastService. This bug/issue was introduced since 8.0.0. I can workaround it by specifying an empty object, but a check on null would be more developer friendly.
2. The 'error' specifies the ToastType. Apparently it's just being added as a class to the toast notification. Previously, in the 8.0.0 release, this would be handled correctly with the appropriate md-toast- prefix. In the 8.1.1 release I have to specify this manually.

To make this work in 8.1.1 I now have to do: show('This is an error message', null, {}, 'md-toast-error'). Can these two issues/differences be updated to how they were in 8.0.0? I'd like my service logic to not care about presentation/CSS classes and for the library to have defensive checks for null. Thanks!

Resources (screenshots, code snippets etc.)