bugfix subset?

Before we had:
(1) -> SI ==> Set Integer
(2) -> SSI ==> Set SI
(3) -> si:SI := set [2]
   (3)  {2}
(4) -> subset?(si,si)
   (4)  true
(5) -> ssi: SSI := set [si]
   (5)  {{2}}
(6) -> subset?(ssi,ssi)
   (6)  false

With the patch also (6) gives true.

git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1781 b0c55286-4f34-0410-a049-a1e7e93b0762