Make guide_bins() respect override.aes

[yutannihilation]

Fix #4085

It seems we simply forgot to pass override.aes to the guide object. guide_merge.bins() and guide_geom.bins() seem to handle override.aes() properly.

devtools::load_all("~/repo/ggplot2")
#> Loading ggplot2

ggplot(mtcars, aes(x = drat, y = wt, size = hp, fill = cyl)) + 
  geom_point(shape = 21) + 
  scale_size_binned(guide = guide_bins(override.aes = list(colour = "red", shape = 21, fill = "blue")))

^{Created on 2020-06-30 by the reprex package (v0.3.0)}

[thomasp85]

LGTM - Thanks!