Restore time-of-day subset begin > end behavior

This restores the behavior that existed before the prior commit to
improve performance.

Yes, this actually makes sense and is useful for financial markets.
For example, some futures markets open at 18:00 and close at 16:00 the
next day.

See #193.

R/xts.methods.R

@@ -34,7 +34,12 @@
   secBegin <- timestringToSeconds(fromTimeString)
   secEnd   <- timestringToSeconds(toTimeString)
 
-  which(secOfDay >= secBegin & secOfDay <= secEnd)
+  if (secBegin <= secEnd) {
+    i <- secOfDay >= secBegin & secOfDay <= secEnd
+  } else {
+    i <- secOfDay >= secBegin | secOfDay <= secEnd
+  }
+  which(i)
 }
 
 .subset_xts <- function(x, i, j, ...) {

inst/unitTests/runit.subset.R

@@ -318,6 +318,16 @@ test.time_of_day_start_equals_end <- function() {
   checkIdentical(.index(x["T01:00/T01:00"]), i1)
 }
 
+test.time_of_day_end_before_start <- function() {
+  # Yes, this actually makes sense and is useful for financial markets
+  # E.g. some futures markets open at 18:00 and close at 16:00 the next day
+  i <- 0:47
+  x <- .xts(i, i * 3600, tz = "UTC")
+  i1 <- .index(x[-c(18L, 42L)])
+
+  checkIdentical(.index(x["T18:00/T16:00"]), i1)
+}
+
 # TODO: Add tests for possible edge cases and/or errors
 # end time before start time
 # start time and/or end time missing "T" prefix