Update index if tclass set to non-TZ from TZ class

Updating the tclass from a class with a timezone to one without a
timezone did not actually update the index. This meant that converting
a POSIXct index to a Date index wouldn't actually change the index to
a POSIXct value at midnight. Thanks to Daniel Palomar for the report.

Reverse dependency checks passed after making this change.

Fixes #311.

R/tclass.R

@@ -124,6 +124,12 @@ function(x, value) {
   }
   attr(attr(x,'index'), 'tclass') <- value
 
+  x_has_tz <- !isClassWithoutTZ(x)
+  if(x_has_tz && isClassWithoutTZ(value)) {
+    # update index values to midnight UTC (this also changes the tzone)
+    index(x) <- index(x)
+  }
+
   # Remove class attrs (object created before 0.10-3)
   attr(x, ".indexCLASS") <- NULL
   attr(x, "tclass") <- NULL

inst/tinytest/test-tclass.R

@@ -65,3 +65,9 @@ info_msg <- "tclass() on object with no tclass/.indexCLASS returns POSIXct"
 x <- structure(1:5, .Dim = c(5L, 1L), index = 1:5, class = c("xts", "zoo"))
 expect_warning(xtc <- tclass(x), "index does not have a 'tclass' attribute")
 expect_identical(c("POSIXct", "POSIXt"), xtc)
+
+info_msg <- "tclass<-() updates index"
+x <- xts(1, .POSIXct(14400, tz = "Europe/Berlin"))
+tclass(x) <- "Date"
+expect_identical(as.numeric(.index(x)), 0, info = paste(info_msg, "values"))
+expect_identical(tzone(x), "UTC", info = paste(info_msg, "timezone"))