Make zero-length subset return same storage type

Subsetting a zero-length xts object always returned a "logical" xts
object, regardless of the input type.

Fixes #376.

R/xts.methods.R

@@ -193,7 +193,9 @@ function(x, i, j, drop = FALSE, which.i=FALSE,...)
         i <- seq_len(nr)
 
       if(length(x)==0) {
-        x.tmp <- .xts(rep(NA, length(i)), .index(x)[i], tclass(x), tzone(x),
+        cdata <- rep(NA, length(i))
+        storage.mode(cdata) <- storage.mode(x)
+        x.tmp <- .xts(cdata, .index(x)[i], tclass(x), tzone(x),
                       dimnames = list(NULL, colnames(x)))
         return(x.tmp)
       } else {

inst/unitTests/runit.subset.R

@@ -325,3 +325,27 @@ test.zero_length_subset_xts_returns_same_tclass <- function() {
   checkTrue(tclass(x[0,]) == "POSIXct")
   checkTrue(tzone(x[0,]) == "America/Chicago")
 }
+
+test.zero_length_subset_returns_same_storage_mode <- function() {
+  # integer
+  x <- .xts(matrix(integer(0), 0), integer(0))
+  checkEquals(storage.mode(x), storage.mode(x[0, ]))
+  checkEquals(storage.mode(x), storage.mode(x[0, 0]))
+  checkEquals(storage.mode(x), storage.mode(x[0, FALSE]))
+
+  x <- .xts(matrix(integer(0), 0, 2), integer(0))
+  checkEquals(storage.mode(x), storage.mode(x[0,]))
+  checkEquals(storage.mode(x), storage.mode(x[0, 1]))
+  checkEquals(storage.mode(x), storage.mode(x[0, c(TRUE, FALSE)]))
+
+  # numeric
+  x <- .xts(matrix(numeric(0), 0), integer(0))
+  checkEquals(storage.mode(x), storage.mode(x[0, ]))
+  checkEquals(storage.mode(x), storage.mode(x[0, 0]))
+  checkEquals(storage.mode(x), storage.mode(x[0, FALSE]))
+
+  x <- .xts(matrix(numeric(0), 0, 2), integer(0))
+  checkEquals(storage.mode(x), storage.mode(x[0,]))
+  checkEquals(storage.mode(x), storage.mode(x[0, 1]))
+  checkEquals(storage.mode(x), storage.mode(x[0, c(TRUE, FALSE)]))
+}