zoomToFit should allow image to scale up to full parent size

If the user overrides `object-fit` from `scale-down` to `contain` like:
```css
.image-view .zoom-to-fit.image-container img {
  object-fit: contain;
}
```

then before this change an svg image (atom#165) wouldn't scale up beyond its
default size. After this change an svg image does scale up to fill the
whole window.

lib/image-editor-view.js

@@ -165,8 +165,8 @@ export default class ImageEditorView {
     this.mode = 'zoom-to-fit'
     this.refs.imageContainer.classList.add('zoom-to-fit')
     this.refs.zoomToFitButton.classList.add('selected')
-    this.refs.image.style.width = ''
-    this.refs.image.style.height = ''
+    this.refs.image.style.width = '100%'
+    this.refs.image.style.height = 'auto'
     this.refs.resetZoomButton.textContent = 'Auto'
     this.percentageStep = 4
   }