emr_track.mv does not copy track attributes

[nettam]

See the following example:

emr_track.create('test', categorical=FALSE, iterator='patients.dob', expr='patients.dob')
#' NULL
emr_track.attr.set('test', 'source', 'dod')
emr_track.attr.export('test')
#'  track   attr value
#' 1  test source   dod
emr_track.var.set('test', 'stam', 1)
emr_track.var.get('test', 'stam')
#' [1] 1
emr_track.mv('test', 'another_test')
emr_track.var.ls('another_test')
#' [1] "stam"
emr_track.attr.export('another_test')
#' [1] track attr  value
#' <0 rows> (or 0-length row.names)