Ensure http date is generated in ASCII

See comment here:

https://github.com/rstudio/shinyapps/commit/8cd34fd04175a42c110a2f3a8b800c68af9445c5#commitcomment-6732864
It is better practice to restore settings via on.exit(), to make sure the settings are always restored even if errors occur in the following code.

    on.exit(Sys.setlocale("LC_TIME", loc), add = TRUE)