fixed a bug in map for sets

tests/stdlib/tsets.nim

@@ -1,13 +1,17 @@
 discard """
-  output: '''true'''
+  output: '''true
+true'''
 """
 
 import sets
 var
   a = initSet[int]()
   b = initSet[int]()
+  c = initSet[string]()
 
 for i in 0..5: a.incl(i)
 for i in 1..6: b.incl(i)
+for i in 0..5: c.incl($i)
 
 echo map(a, proc(x: int): int = x + 1) == b
+echo map(a, proc(x: int): string = $x) == c