Set .ROWNAMES attr before dropping row names

The previous commit removed row names from the object returned by
.xts(), but it did not set the .ROWNAMES attribute. This broke several
tests that attempted to reclass objects back to their original class.

This commit fixes those tests, while preserving the behavior that
removes row names from the object returned by .xts().

Fixes #298. (for real)

R/xts.R

@@ -192,7 +192,9 @@ function(x=NULL, index, tclass=c("POSIXct","POSIXt"),
               c('xts','zoo'), tformat, PACKAGE='xts')
 
   # ensure there are no rownames
-  if(!is.null((dimnames(xx)))) {
+  rn <- dimnames(xx)[[1]]
+  if(!is.null(rn)) {
+    attr(xx, '.ROWNAMES') <- rn
     dimnames(xx)[[1]] <- NULL
   }