title: "unlist" removes class in list elements (POSIXlt)
body: <p>I have an object that is a multi-level list, where all elements are of class <code>POSIXlt</code>:</p>
  
  <pre><code>[[1]]
   [1] "2015-07-16 GMT" "2015-07-16 GMT" "2015-07-16 GMT" "2015-07-16 GMT" "2015-07-15 GMT"
   [6] "2015-07-15 GMT" "2015-07-15 GMT" "2015-07-15 GMT" "2015-07-15 GMT" "2015-07-14 GMT"
  [11] "2015-07-14 GMT" "2015-07-14 GMT" "2015-07-14 GMT" "2015-07-14 GMT" "2015-07-14 GMT"
  [16] "2015-07-14 GMT" "2015-07-14 GMT" "2015-07-14 GMT" "2015-07-14 GMT" "2015-07-14 GMT"
  [21] "2015-07-13 GMT" "2015-07-13 GMT" "2015-07-13 GMT" "2015-07-13 GMT" "2015-07-13 GMT"
  [26] "2015-07-13 GMT" "2015-07-13 GMT" "2015-07-13 GMT" "2015-07-13 GMT"
  
  [[2]]
   [1] "2015-07-12 GMT" "2015-07-11 GMT" "2015-07-11 GMT" "2015-07-11 GMT" "2015-07-11 GMT"
   [6] "2015-07-10 GMT" "2015-07-10 GMT" "2015-07-10 GMT" "2015-07-09 GMT" "2015-07-09 GMT"
  [11] "2015-07-09 GMT" "2015-07-09 GMT" "2015-07-09 GMT" "2015-07-09 GMT" "2015-07-09 GMT"
  [16] "2015-07-09 GMT" "2015-07-08 GMT" "2015-07-08 GMT" "2015-07-08 GMT" "2015-07-08 GMT"
  [21] "2015-07-08 GMT" "2015-07-08 GMT" "2015-07-08 GMT" "2015-07-07 GMT" "2015-07-07 GMT"
  [26] "2015-07-07 GMT" "2015-07-07 GMT" "2015-07-07 GMT" "2015-07-06 GMT"
  
  [[3]]
   [1] "2015-07-06 GMT" "2015-07-06 GMT" "2015-07-06 GMT" "2015-07-06 GMT" "2015-07-06 GMT"
   [6] "2015-07-06 GMT" "2015-07-06 GMT" "2015-07-06 GMT" "2015-07-06 GMT" "2015-07-05 GMT"
  [11] "2015-07-05 GMT" "2015-07-05 GMT" "2015-07-05 GMT" "2015-07-05 GMT" "2015-07-04 GMT"
  [16] "2015-07-04 GMT" "2015-07-04 GMT" "2015-07-04 GMT" "2015-07-03 GMT" "2015-07-03 GMT"
  [21] "2015-07-03 GMT" "2015-07-03 GMT" "2015-07-03 GMT" "2015-07-03 GMT" "2015-07-03 GMT"
  [26] "2015-07-03 GMT" "2015-07-02 GMT" "2015-07-02 GMT" "2015-07-02 GMT"
  </code></pre>
  
  <p>When I try to make a single vector with <code>unlist</code>, the object is converted to <code>numeric</code> (i.e. with <code>unlist(dates)</code>). How can I maintain a POSIXlt class?</p>
  
  <h3>Example data:</h3>
  
  <pre><code>dates &lt;- list(structure(list(sec = c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0), min = c(0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L), hour = c(0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L), mday = c(16L, 
  16L, 16L, 16L, 15L, 15L, 15L, 15L, 15L, 14L, 14L, 14L, 14L, 14L, 
  14L, 14L, 14L, 14L, 14L, 14L, 13L, 13L, 13L, 13L, 13L, 13L, 13L, 
  13L, 13L), mon = c(6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 
  6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 
  6L, 6L), year = c(115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 
  115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 
  115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L), 
      wday = c(4L, 4L, 4L, 4L, 3L, 3L, 3L, 3L, 3L, 2L, 2L, 2L, 
      2L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
      1L, 1L), yday = c(196L, 196L, 196L, 196L, 195L, 195L, 195L, 
      195L, 195L, 194L, 194L, 194L, 194L, 194L, 194L, 194L, 194L, 
      194L, 194L, 194L, 193L, 193L, 193L, 193L, 193L, 193L, 193L, 
      193L, 193L), isdst = c(0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
      0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
      0L, 0L, 0L, 0L, 0L)), .Names = c("sec", "min", "hour", "mday", 
  "mon", "year", "wday", "yday", "isdst"), class = c("POSIXlt", 
  "POSIXt"), tzone = "GMT"), structure(list(sec = c(0, 0, 0, 0, 
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
  0, 0, 0, 0), min = c(0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L), hour = c(0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L), mday = c(12L, 11L, 11L, 11L, 11L, 10L, 10L, 10L, 
  9L, 9L, 9L, 9L, 9L, 9L, 9L, 9L, 8L, 8L, 8L, 8L, 8L, 8L, 8L, 7L, 
  7L, 7L, 7L, 7L, 6L), mon = c(6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 
  6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 
  6L, 6L, 6L, 6L, 6L), year = c(115L, 115L, 115L, 115L, 115L, 115L, 
  115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 
  115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 
  115L), wday = c(0L, 6L, 6L, 6L, 6L, 5L, 5L, 5L, 4L, 4L, 4L, 4L, 
  4L, 4L, 4L, 4L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 2L, 2L, 2L, 2L, 2L, 
  1L), yday = c(192L, 191L, 191L, 191L, 191L, 190L, 190L, 190L, 
  189L, 189L, 189L, 189L, 189L, 189L, 189L, 189L, 188L, 188L, 188L, 
  188L, 188L, 188L, 188L, 187L, 187L, 187L, 187L, 187L, 186L), 
      isdst = c(0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
      0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
      0L, 0L)), .Names = c("sec", "min", "hour", "mday", "mon", 
  "year", "wday", "yday", "isdst"), class = c("POSIXlt", "POSIXt"
  ), tzone = "GMT"), structure(list(sec = c(0, 0, 0, 0, 0, 0, 0, 
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
  0), min = c(0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L
  ), hour = c(0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L
  ), mday = c(6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 5L, 5L, 5L, 5L, 
  5L, 4L, 4L, 4L, 4L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 2L, 2L, 2L
  ), mon = c(6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 
  6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L
  ), year = c(115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 
  115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 
  115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L, 115L), wday = c(1L, 
  1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 0L, 0L, 0L, 0L, 0L, 6L, 6L, 6L, 
  6L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 5L, 4L, 4L, 4L), yday = c(186L, 
  186L, 186L, 186L, 186L, 186L, 186L, 186L, 186L, 185L, 185L, 185L, 
  185L, 185L, 184L, 184L, 184L, 184L, 183L, 183L, 183L, 183L, 183L, 
  183L, 183L, 183L, 182L, 182L, 182L), isdst = c(0L, 0L, 0L, 0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 
  0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L)), .Names = c("sec", "min", 
  "hour", "mday", "mon", "year", "wday", "yday", "isdst"), class = c("POSIXlt", 
  "POSIXt"), tzone = "GMT"))
  </code></pre>