Use daily periodicity for getSymbols.tiingo()

We were using a POSIXct index for the results, while other getSymbols
methods use a Date index for daily data. And tiingo only provides
daily end of day data anyway.

We could convert to weekly, monthly, or yearly. But that functionality
doesn't exist now, so no one could be using it. And users can convert
to higher periodicity after the call to getSymbols().

Fixes #350.

R/getSymbols.R

@@ -1486,7 +1486,7 @@ getSymbols.tiingo <- function(Symbols, env, api.key,
   if (!hasArg("verbose")) verbose <- FALSE
   if (!hasArg("warnings")) warnings <- TRUE
 
-  valid.periodicity <- c("daily", "weekly", "monthly", "annually")
+  valid.periodicity <- "daily"
   periodicity <- match.arg(periodicity, valid.periodicity)
   default.return.class <- return.class
   default.periodicity <- periodicity
@@ -1526,7 +1526,8 @@ getSymbols.tiingo <- function(Symbols, env, api.key,
       stop(msg, call. = FALSE)
     }
 
-    tm.stamps <- as.POSIXct(stock.data[, "date"], ...)
+    tm.stamps <- as.Date(stock.data[, "date"])
+
     if (adjust) {
       stock.data <- stock.data[, c("adjOpen", "adjHigh", "adjLow", "adjClose", "adjVolume")]
     } else {