at2x mixin doesn't support single keyword values for background-size

[alexgibson]

Description

It should be possible to use the at2x mixin with background-size single keyword values, such as cover and contain.

Steps to reproduce

@include at2x('/path/to/some/background-image.png', cover);

Expected result

background-size: cover;

Actual result

background-size: cover auto; which is invalid.

Environment

Firefox Nightly 97
macOS

[janbrasna]

Only special-cased values are checked to skip emitting auto as the second param, contain should be supported since #336 v6.0.1 and is used around bedrock successfully; I'll however add cover as the other case with such treatment for consistency.