title: R, lag( ) has inconsistent behavior for xts and ts objects
body: <p>I would like to take a lag of an xts variable, and the lag() function returns a lag. However, if I use it on a ts variable, it gives a lead. Is this a bug, or working as intended?</p>
  
  <pre><code>library('xts')
  
  a = as.xts(ts(c(5,3,7,2,4,8,3), start=c(1980,1), freq=4))
  cbind(a, lag(a)) # provides lag 1
  #         ..1 ..2
  # 1980 Q1   5  NA
  # 1980 Q2   3   5
  # 1980 Q3   7   3
  # 1980 Q4   2   7
  # 1981 Q1   4   2
  # 1981 Q2   8   4
  # 1981 Q3   3   8
  b = ts(c(5,3,7,2,4,8,3), start=c(1980,1), freq=4)
  cbind(b, lag(b)) # provides lead 1
  #          b lag(b)
  # 1979 Q4 NA      5
  # 1980 Q1  5      3
  # 1980 Q2  3      7
  # 1980 Q3  7      2
  # 1980 Q4  2      4
  # 1981 Q1  4      8
  # 1981 Q2  8      3
  # 1981 Q3  3     NA
  </code></pre>