Unname the result of manual_pal()

[yutannihilation]

(Originally discussed at tidyverse/ggplot2#4087 (comment))

ggplot2's scale_manual() family uses two types of mapping colours to values; by position and by name. manual_pal() is meant to be used for the latter one. But, since the palette manual_pal() provides aren't unnamed, it might be accidentally mapped in by-name manner (See tidyverse/ggplot2#4087 for an example). So, I think it should be unname()ed as the names are not useful anyway, I believe.

If this looks good, I'll create a pull request for this.

[hadley]

Yeah, that sounds good to me.

[yutannihilation]

Thanks, here it is: #291