Normalize zero length and percentage properties.

[tkaemming]

This was pointed out in GH-24 by @BYK here:

You may wanna have another test case to cove

{
    'margin-top': '0px',
    'margin-right': '2px',
    'margin-bottom': '0',
    'margin-left': '2px',
}

Basically when the value is 0, you should still collapse that regardless of the unit.