fixed minor bugs; cleaned up tests

tests/collections/tindexby.nim

@@ -0,0 +1,22 @@
+import tables
+
+doAssert indexBy(newSeq[int](), proc(x: int):int = x) == initTable[int, int](), "empty int table"
+
+var tbl1 = initTable[int, int]()
+tbl1.add(1,1)
+tbl1.add(2,2)
+doAssert indexBy(@[1,2], proc(x: int):int = x) == tbl1, "int table"
+
+type
+  TElem = object
+    foo: int
+    bar: string
+    
+let
+  elem1 = TElem(foo: 1, bar: "bar")
+  elem2 = TElem(foo: 2, bar: "baz")
+  
+var tbl2 = initTable[string, TElem]()
+tbl2.add("bar", elem1)
+tbl2.add("baz", elem2)
+doAssert indexBy(@[elem1,elem2], proc(x: TElem): string = x.bar) == tbl2, "element table"