Fixed subsasgn method (when calling merge).

It was no more possible to fill a variable with NaNs, after the switch
to the new behaviour of the merge method (see test #5 in
@dseries/merge.m). Added a legacy mode to the merge method to
circumvent the issue (@dseries/subsasgn sets legacy equal to true when
calling the merge method).

(cherry picked from commit 67077fe8)