Due to the Javascript modulo bug
(https://stackoverflow.com/questions/4467539/javascript-modulo-gives-a-negative-result-for-negative-numbers),
any negative value for hue results in a bug where the resulting color will be black. Negative hues
should cycle through to 359, so that functions like `adjustHue()` can provide consistent behavior
for all colors.

fix styled-components#437

Since the % operation is already performed inside `hslToRgb()` in `src/internalHelpers/_hslToRgb.js`
(which `adjustHue()` eventually calls through `toColorString()`), this % is completely redundant.

refactor styled-components#437

Test hslToRgb's ability to handle negative hues passed in through adjustHue.

tests styled-components#437