Nested objects with GeoJSON will not after setFeatureState

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Hi!

**mapbox-gl-js version**: v0.48

**browser**: Chrome 68.0.3440.106

### Steps to Trigger Behavior

Playing around with the hover-styles example (https://www.mapbox.com/mapbox-gl-js/example/hover-styles/), I encountered an issue that nested objects in a features object will not work after setFeatureState is called.

More specifically, doing this will work fine on initial rendering.
```
"line-color":
["case", ["boolean", ["feature-state", "hover"], false],
["get", "default-color"],
["get", "0", ["get", "colors"]]
],
```
However, after a `mouseover` and `mouseleave`, it will however revert to using the default fallback line color instead of the one retrieved with ` ["get", "0", ["get", "colors"]]`.

Additionally, this warning will be logged to the console:
`Expected value to be of type object, but found string instead.`

### Link to Demonstration

https://jsfiddle.net/43bq7mfg/

### Expected Behavior

Using `get` on nested objects in feature properties to work after using `setFeatureState()`

### Actual Behavior

Using `get` on nested objects in feature properties logs a warning and will revert to the fallback/default value.

I'm not sure, but could this be related to #2434?

Start by reproducing the issue in the hover-styles example or the linked JSFiddle, focusing on setFeatureState, feature-state, and nested get expressions. Done means nested GeoJSON properties still resolve after mouseover and mouseleave, without the reported type warning or fallback color.

Written by the indexing model from the issue text.