See #703 - re-enables `background` longhand merging.

See #703 - re-enables `background` longhand merging.

Why:

* See #577 and #579 - these two clearly state the issue is about
  IE8 not IE9+, and according to tests and caniuse.com tables
  using `background-(clip|origin|size)` in `background` shorthand
  is supported on IE9+;
* At this point I have no idea why it was disabled at the time.
  One idea is that it was disabled because IE9 doesn't support
  `properties.spaceAfterClosingBrace` and these two issues
  overlappedi at the time of 3.2 branch. (both were "fixed" in 3.3);
* Let's enable it again, since the default mode is now IE10+.

** Important note **: Safari <7 and Android stock browser <4.4
does not support it either so you'll likely have to disable if
you still support those. Just use `--compatibility ie8` to do so.

Test case: https://jsfiddle.net/7zLwv7o0/5/