fix "series" of UnivariateLaurentSeriesConstructor

Q ==> Fraction Integer
R ==> Record(k: Integer, c: Q)
L ==> UnivariateLaurentSeries(Q, 'z, 0)
l: List R := [[n, n/1]$R for n in 1..7|n~=5]
series(l)$L

Result:
  before
                  2     3     4
            z + 2z  + 3z  + 4z

  after
                  2     3     4     6     7
            z + 2z  + 3z  + 4z  + 6z  + 7z

git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1833 b0c55286-4f34-0410-a049-a1e7e93b0762