prevent margin from changing the unit class when subclassing

[thomasp85]

I've uncovered another "bad" assumption in ggplot2's handling of units. The margin constructor will drop any unit subclasses when subclassing the unit it creates. This PR fixes it and makes it work with the experimental unit implementation in thomasp85/grid#1 in a back-compatible way

[yutannihilation]

Travis says (https://api.travis-ci.org/v3/job/502563012/log.txt):

* checking for future file timestamps ... WARNING
unable to verify current time

This warning comes from here:

                now <- tryCatch({
                    foo <- readLines("http://worldclockapi.com/api/json/utc/now",
                                     warn = FALSE)
                    as.POSIXct(gsub(".*\"currentDateTime\":\"([^Z]*).*", "\\1", foo),
                               "UTC", "%Y-%m-%dT%H:%M")
                }, error = function(e) NA)
                if (is.na(now)) {
                    any <- TRUE
                    warningLog(Log, "unable to verify current time")

But this seems unlikely to success... Probably we can ignore.

readLines("http://worldclockapi.com/api/json/utc/now",
                                     warn = FALSE)
#> Warning in file(con, "r"): cannot open URL 'http://worldclockapi.com/api/
#> json/utc/now': HTTP status was '403 Forbidden'
#> Error in file(con, "r"): cannot open the connection to 'http://worldclockapi.com/api/json/utc/now'

^{Created on 2019-03-07 by the reprex package (v0.2.1.9000)}

[yutannihilation]

I restarted the job. Let's see what will happen. If this consistently fails, it seems better to set _R_CHECK_SYSTEM_CLOCK_ to false.

c.f. https://twitter.com/krlmlr/status/1084496032187199488

[thomasp85]

@yutannihilation this is due to upstream problems, not something in ggplot2

[yutannihilation]

@thomasp85 Yeah, could you approve #3179?

[lock]

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