[Rd] pmin/pmax issue with ordered factors

[Erwan Le Pennec]

     Dear all,

pmin/pmax used to work with ordered factors but fail now since this 
summer when those functions have tried to handle more cases.

A simple way to trigger the issue is:

 > min(ordered(c(1,5,6)))
[1] 1
Levels: 1 < 5 < 6
 > pmin(ordered(c(1,5,6)), ordered(c(1,5,6)))
Error in `mostattributes<-`(`*tmp*`, value = attributes(elts[[1L]])) :
   adding class "factor" to an invalid object

A simple fix is to explicitly test for the ordered class and use the 
internal method in that case as proposed in the attached patch.

     Yours,

         Erwan

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