Fix HSL alpha channel bug.

[ben-eb]

Strange error when parsing a colour such as hsla(0, 0%, 0%, 0) - alpha channel gets converted to 1. Fixed this in both the HSL and HWB conversion functions, such that this test now passes:

assert.deepEqual(string.getHsla('hsla(0,0%,0%,0)'), [0, 0, 0, 0]);