take care that input really are integer, not only practically

github issue #259: backtransformation to integers can be inexact,
but analysis assumes exact integers because input are truncated.
Now we first check that the values are practically integers, and
then silently take care that they are exactly integers with round().

(cherry picked from commit 87d76f0)

R/rrarefy.R

@@ -6,6 +6,8 @@
     if (!identical(all.equal(x, round(x)), TRUE))
         stop("function is meaningful only for integers (counts)")
     x <- as.matrix(x)
+    ## x may not be exactly integer, since, e.g., sqrt(2)^2 != 2
+    x <- round(x)
     if (ncol(x) == 1)
         x <- t(x)
     if (length(sample) > 1 && length(sample) != nrow(x))