Stop resolution multiplying rows from metadata

[jameslairdsmith]

(Fixes #379)

The metadata column was causing certain rows to recycle over the result data frame. This could happen with any list column that isn't of length one. The solution I created here is to check if the list is not already length one and to wrap it into a list if it is not.

Also added an assertion to force the output into being one row as desired.

Very happy to accept changes as you think is appropriate.

[gaborcsardi]

Thanks! I don't love my old solution of magically fixing bad resolution output. But if we are already doing that, we might as well do it properly.

It seems that

      res$metadata[["RemoteRepos"]] <- config$get("cran-mirror")

converts the metadata character vector to a list silently, that's how we end up with a list.

I am not sure why config$get("cran-mirror") is a list, this can possibly cause other problems as well, so I'll fix that as well.

[gaborcsardi]

Thanks again!