[Bug] Fixed Background Image doesn't work

[jessecowens]

Describe the bug
The "Fixed" option for the background selected in the Customize -> Background scrolls.

To Reproduce
Steps to reproduce the behavior:

1. Go to Customize -> Background
2. Select a background image and the "scroll" property
3. Image still scrolls both in Customizer and on front-end

Expected behavior
The background should remain fixed when the user scrolls

Screenshots