Treat NA the same as NULL for start or end values

NULL represents an undefined index value. NA represents an unknown or 
missing index value. xts does not allow NA as index values. Subsetting 
an xts or zoo object by NA returns a zero-length object. So a NA 
(unknown) index value is essentially the same as an undefined index 
value.

Closes #383. Fixes #345.

R/xts.methods.R

@@ -396,6 +396,10 @@ window_idx <- function(x, index. = NULL, start = NULL, end = NULL)
 # that is, index. must be time based,
 window.xts <- function(x, index. = NULL, start = NULL, end = NULL, ...)
 {
+  # scalar NA values are treated as NULL
+  if (isTRUE(is.na(start))) start <- NULL
+  if (isTRUE(is.na(end))) end <- NULL
+
   if(is.null(start) && is.null(end) && is.null(index.)) return(x)
 
   # dispatch to window.zoo() for yearmon and yearqtr

inst/tinytest/test-subset.R

@@ -276,6 +276,9 @@ w2 <- window(x2, start = "2015Q1")      # to window.zoo()
 expect_equal(r1, w1, info = paste(info_msg, "window, yearmon, character start"))
 expect_equal(r2, w2, info = paste(info_msg, "window, yearqtr, character start"))
 
+w1 <- window(x1, start = "2015-01-01", end = NA)  # to window.xts()
+expect_equal(r1, w1, info = paste(info_msg, "window, yearmon, character start with end = NA"))
+
 info_msg <- "test.zero_width_subset_does_not_drop_class"
 target <- c("custom", "xts", "zoo")
 x <- .xts(1:10, 1:10, class = target)

inst/tinytest/test-xts.methods.R

@@ -150,6 +150,41 @@ bin <- window(x)
 reg <- window_dbg(x, start = start, end = end)
 expect_identical(bin, reg, info = paste(info_msg, "- end = NULL, start = NULL"))
 
+# Test just start, end = NA
+start <- base + 13 * DAY
+end <- base + 30*DAY
+bin <- window(x, start = start, end = NA)
+reg <- window_dbg(x, start = start, end = end)
+expect_identical(bin, reg, info = paste(info_msg, "- just start, end = NA"))
+
+# Test just start, end = NA, empty range
+start <- base + 25 * DAY
+end <- base + 30*DAY
+bin <- window(x, start = start, end = NA)
+reg <- window_dbg(x, start = start, end = end)
+expect_identical(bin, reg, info = paste(info_msg, "- just start, end = NA, empty range"))
+
+# Test just end, start = NA
+end <- base + 13 * DAY
+start <- base
+bin <- window(x, start = NA, end = end)
+reg <- window_dbg(x, start = start, end = end)
+expect_identical(bin, reg, info = paste(info_msg, "- just end, start = NA"))
+
+# Test just end, start = NA, empty range
+end <- base
+start <- base
+bin <- window(x, start = NA, end = end)
+reg <- window_dbg(x, start = start, end = end)
+expect_identical(bin, reg, info = paste(info_msg, "- just end, start = NA, empty range"))
+
+# Test end = NA, start = NA
+start <- base
+end <- base + 30*DAY
+bin <- window(x, start = NA, end = NA)
+reg <- window_dbg(x, start = start, end = end)
+expect_identical(bin, reg, info = paste(info_msg, "- end = NA, start = NA"))
+
 #######################################
 # Test for index. parameter
 start <- base