fix s3method dispatch unit test fail (#3351)

NAMESPACE

@@ -113,11 +113,11 @@ import(stats)
 S3method(na.omit, data.table)
 
 S3method(as.data.table, xts)
-if(getRversion() >= "3.6.0") {
+if (getRversion() >= "3.6.0") {
   export(as.xts.data.table)          # fails in R-devel if not exported too, but I don't understand why
   S3method(xts::as.xts, data.table)  # delayed registration (new in R-devel)  -- shouldn't this also export as.xts.data.table for us?
 } else {
-  export(as.xts.data.table)
+  export(as.xts.data.table)          # this on some configuration happens to fail as.xts dispatch to dt method #3286
 }
 
 # IDateTime support:

inst/tests/tests.Rraw

@@ -6356,6 +6356,7 @@ if (test_xts) {
   test(1465.10, as.data.table(xt), error = 'Input xts object should not')
   names(xt)[1L] = 'quantity'
   setcolorder(dt, c(3, 1, 2))
+  if (base::getRversion() < "3.6.0") as.xts = as.xts.data.table # fix for when we cannot register s3method for suggested dependency #3286
   test(1465.11, as.xts(dt), error = 'data.table must have a time based')
   setcolorder(dt, c(2, 3, 1))
   dt[ , char_col := 'a']
@@ -6364,8 +6365,6 @@ if (test_xts) {
   Sys.setenv("_R_CHECK_LENGTH_1_LOGIC2_" = TRUE)
 }
 
-
-
 # as.data.table.default #969
 ar <- array(NA, dim=c(10,4),dimnames = list(NULL,paste("col",1:4,sep="")))
 test(1466.1, as.data.table(as.data.frame(ar)), as.data.table(ar)) # array type