'UTC' equality can be extended to other equivalent time zones

[MichaelChirico]

I just filed this at data.table: Rdatatable/data.table#4117

And this fix for base R: https://bugs.r-project.org/bugzilla/show_bug.cgi?id=17674

I notice also several usages of == 'UTC' or != 'UTC' in lubridate:

lubridate/R/parse.r:679:    if (tz == "UTC"){
lubridate/R/parse.r:703:    if (tz == "UTC") {
lubridate/R/parse.r:759:    if (tz != "UTC") {
lubridate/R/parse.r:802:      if (tz != "UTC"){
lubridate/R/stamp.r:110:          if (tz(x[[1]]) != "UTC")
lubridate/R/util.r:119:  if (tz == "UTC") utc
lubridate/R/POSIXt.r:35:  t0 <-if (tz == "UTC") origin else force_tz(origin, tz)

If you're amenable, I would file a PR to an equivalent is_utc internal utility & replace the above instances

[vspinu]

Yes, please.