getStyle then setStyle broken for raster-dem

Nobody has claimed this yet.

**mapbox-gl-js version**: tested on 1.1.0, probably since forever

**browser**: Chrome 75.0.3770.100 (Official Build) (64-bit)

### Steps to Trigger Behavior

1. setStyle on a style with raster-dem source
2. getStyle anytime
3. setStyle with serialized style object from getStyle

### Link to Demonstration
https://jsfiddle.net/srmanc/L20ha5um/1/

### Expected Behavior
setStyle should work without error msgs

### Actual Behavior
setStyle breaks because bounds and url are set on source but are undefined. Sometimes tiles is undefined on source as well.
There are actually two issues:
1. Why does setStyle break at all if some undefined properties are set on source (this only happens for raster-dem type).
2. When is it acceptable to call getStyle() to get all the relevant properties on sources - they are set initially so there would be no point in not setting them right away - however, tiles property is missing at the first "styledata" event - please review the jsfiddle example with console.log msgs for more input. We can make this work using delete operator but this seems like a nasty bug.

Reproduce the failure using the linked JSFiddle and the setStyle/getStyle sequence with a raster-dem source. Inspect how source properties are serialized and restored, including bounds, url, and tiles, and consider both reported questions; done means a style returned by getStyle can be passed back to setStyle without errors or missing required source data.

Written by the indexing model from the issue text.