scl should not write into colorscale when both defined 

Follow up of https://github.com/plotly/plotly.js/pull/3418#discussion_r246056443 and as illustrated in [this demo](https://codepen.io/MojtabaSamimi/pen/VqdoBy), `colorscale` & `reversescale` would be ignored if `scl` and/or `colorscl` were defined.
