[Rd] suggestion: better support for https CRAN mirrors

[Arthur Vigil]

Hello,
I see that https CRAN mirrors are starting to pop up, as per this post at https://support.rstudio.com/hc/en-us/articles/206827897-Secure-Package-Downloads-for-R

However, trying to use one of these mirrors without changing the default download.file.method option gives me errors

> Warning: unable to access index for repository https://cran.rstudio.com/src/contrib
> Warning: unable to access index for repository https://cran.rstudio.com/bin/macosx/mavericks/contrib/3.2

I don’t mind just choosing an http mirror but when prompted to select a mirror, there is no distinction made in the mirror listing between http and https mirrors. It would be nice if https mirrors were hidden when the current environment doesn’t support them. Alternatively, at least make a distinction between http and https in the mirror listing. They currently just look like duplicates. I think this is a pretty trivial change but would help a lot with usability.

As an example, when using install.packages:

> --- Please select a CRAN mirror for use in this session ---
> CRAN mirror 
> 
>   1: 0-Cloud                        2: 0-Cloud                    
>   3: Algeria                        4: Argentina (La Plata)       
>   5: Australia (Canberra)           6: Australia (Melbourne)      
>   7: Austria                        8: Austria                    
>   9: Belgium                       10: Brazil (BA)                
>  11: Brazil (PR)                   12: Brazil (RJ)                
>  13: Brazil (SP 1)                 14: Brazil (SP 2)    

In the listing above there are duplicate entries for 0-cloud and Austria pointing to http and https versions of the same mirror.