box-shadow compression leading space issue

The edge case will happen when someone is using `box-shadow: 0 0 0 0` instead of  `box-shadow:0 0 0 0`.

`box-shadow: 0 0 0 0` will not be compiled to expected `box-shadow:0 0`

Therefore, `box-shadow: 0 0 0 0` will be compressed following this rule `replace(/:0 0 0 0([^\.])/g, ':0$1');`, which will output invalid `box-shadow: 0`.