Fix geom2trace.geompoint incorrect filling (closes #2298)

[zeehio]

I'm unable right now to paste images here. However the plot p below gives a red and a green dot with ggplot2 and two green dots with plotly.

  df <- data.frame(x = c(1,2), y = c(0,0), color = rep("yellow", 2), fill = 1:2)
  p <- ggplot(df) +
    geom_point(aes(x = x, y = y, fill = fill, color = color), size = 5, shape = 21) +
    scale_fill_gradient(low = "green", high = "red")
  ggplotly(p)

ggplotly(p) gives the following warning as well:

Warning message:
In L$marker$color[idx] <- aes2plotly(data, params, "fill")[idx] :
number of items to replace is not a multiple of replacement length

This PR addresses the warning and fixes the issue. It's fairly simple.

[zeehio]

Rebased conflict on NEWS entry

Thanks for your time and thanks for plotly!

[cpsievert]

Thank you @zeehio, great work!

BTW, if you're looking to contribute more to plotly, it seems some recent changes in the development version of ggplot2 have caused a lot of test failures...it'd be super awesome if you wanted to help surface what changes have lead to those issues (and, ideally, propose fixes for them)!

[zeehio]

I'd love to help. These fixes were part of my work and going beyond this would probably be out of scope for my employer.

But if I find some free personal time I may try to submit some test fixes... Thanks!