RGB(2.0,0.0,0.0) should throw an error

I was surprised that the following different throw an error:
```julia
RGB(2.0,0.0,0.0)
```
This ended up causing an error further down in a plotting package https://github.com/JuliaPlots/Plots.jl/issues/851#issuecomment-302924994. Is this intentional?


@mkborregaard 