Fix marshaling of mixin def node with variable arguments

After seeing this issue for a long time and originally thinking it was an issue in Sprockets (sstephenson/sprockets#536), I've finally narrowed this down to an issue within Sass.

The problem occurs when defining a mixin with a splat argument, and when the engine options are unmarshalable (which is the case with Sprockets). e.g.

```sass
=kaboom($args...)
  color: red
```

which gives an error like:

```
Warning. Error encountered while saving cache 5eeafa3aa0f7f3a490743b762a56c00651a72d7e/test.css.sassc: can't dump anonymous class #<Class:0x007fcacb3ef150>
```

Curiously this does not occur when using SCSS syntax. I'm not sure why.