Make `fast_strptime` consistent with `strptime` internal structure

[vspinu]

library(lubridate, warn.conflicts = F)
fmt <- "%Y%m%d %H:%M:%OS%z"
x <- "20190730 10:00:00+0200"
second(fast_strptime(x, format = fmt))
#> [1] -7200
second(strptime(x, format = fmt, tz = "UTC"))
#> [1] 0

^{Created on 2019-12-03 by the reprex package (v0.3.0)}

Originally reported in #786