Add tclass and class if index has no attr

Some users may set the index using attr<- instead of using the API, as
they should. This may result in a malformed index (e.g. a numeric
vector with no tclass or tzone attributes).

This can cause errors in other xts code. This commit fixes the case
where a call to index() on a malformed xts object throws an error at
the switch() statement, because value[[1]] is "". This case breaks
existing code in the twsInstrument package.

Throw a warning to let users know they're being naughty, and set the
tclass and class to POSIXct to avoid breaking things downstream.

Fixes #297.

R/index.R

@@ -22,8 +22,13 @@
 index.xts <- time.xts <-
 function(x, ...) {
   value <- tclass(x)
-  if(is.null(value))
-    return(.index(x))
+  if(is.null(value) || !nzchar(value)) {
+    warning("index does not have a ", sQuote("tclass"), " attribute\n",
+            "    returning c(\"POSIXct\", \"POSIXt\")")
+    ix <- .index(x)
+    attr(ix, "tclass") <- attr(ix, "class") <- c("POSIXct", "POSIXt")
+    return(ix)
+  }
   #  if tclass is Date, POSIXct time is set to 00:00:00 GMT. Convert here
   #  to avoid ugly and hard to debug TZ conversion.  What will this break? 
   if(value[[1]] == "Date")

inst/unitTests/runit.index.R

@@ -0,0 +1,9 @@
+test.get_index_does_not_error_if_index_has_no_attributes  <- function() {
+  x <- .xts(1:3, 1:3, tzone = "UTC")
+
+  ix <- index(x)
+  ix <- ix + 3
+
+  attr(x, "index") <- 4:6
+  checkEquals(index(x), ix)
+}