[Rd] can't create a POSIXct from an actual Unix timestamp (PR#6975)

[phil at google.com]

Full_Name: Philip Gross
Version: 1.9.0
OS: Linux Redhat 9.0
Submission from: (NULL) (65.57.245.11)


Among the many conversions for POSIXct values, there does not seem to be one
which will accept an actual POSIX timestamp, e.g. as produced by MySQL's
UNIX_TIMESTAMP() function.  I have fixed this by adding the following lines to
R-1.9.0/src/library/base/R/datetime.R , at line 89:

    if(is.numeric(x))
        return(structure(x, class = c("POSIXt", "POSIXct")))

I am far from an R guru, and this was done based on pattern matching the code. 
It seems to work, though.