as_tsibble.ts() for frequencies >24 produces irregular series - use fractional years instead

[mitchelloharawild]

The unit test for as.ts(as_tsibble(x)) == x on EuStockMarkets currently fails. I've removed the test since the time class (POSIXct) for this style of data is inappropriate, which causes the as.ts() method to fill all the gaps of 1s frequency. A more suitable time class for this is fractional years, where 1/260th of a year can be the frequency (e.g. supported by mixtime). I'm creating this issue to track this problem and while the test itself is temporarily removed.

  freq <- frequency(EuStockMarkets)
  x <- head(EuStockMarkets)
  y <- as_tsibble(x)
  expect_equal(
    as.double(as.ts(y, frequency = freq)),
    as.double(x)
  )