Create alias for `colors=` in `scale_color_gradientn()`

[beroe]

Not sure if this has been addressed before, or if it is by design: It works if you spell colours= as an argument for scale_color_gradientn but not if you put colors=

   + scale_color_gradientn(colors = rainbow(4))  # fails      
   + scale_color_gradientn(colours = rainbow(4))  # works

[kellieotto]

Aha. I just had the same problem.