Fix unnecessary DESCRIPTION overwrite (+ unit test)

[renozao]

The changes on R/roclet-collate.R are optional: they only make the function simpler, without the need to write a temporary DESCRIPTION file.

[Geoff99]

Thanks @renozao

Your change to update_collate makes it neater - I didn't like writing out and re-reading the DESCRIPTION file to get the format write - but it was quick and sure to get the same format even if read.description or write.description changed.

I feel much happier having a unit test as well. It doesn't guarantee devtools::load_all will always be safe to run in parallel --- for example if there is a change in the Collate field for the package which is being load_alled, but at least you, as the package owner would have control over that.

I'll submit the merged branch upstream to the official roxygen for Hadley to look over.