fast IDate/Time methods for UTC can be extended to when we can infer tz is UTC

[MichaelChirico]

Currently we test:

if (inherits(x, "POSIXct") && identical(attr(x, "tzone", exact = TRUE), "UTC")) 

However, if 'tzone' attribute is missing and Sys.timezone() is UTC (actually, %chin% c('GMT', 'UTC', 'Etc/UTC', 'Etc/GMT', 'GMT-0', 'GMT+0', 'GMT0') is fine), we can do the same shortcut