Bug fix : ZPOPMIN command.

Fixed the error caused by incorrect "boder" pass value.
And added a simple unit test to verify correctness.

datastruct/sortedset/sortedset_test.go

@@ -0,0 +1,16 @@
+package sortedset
+
+import "testing"
+
+func TestSortedSet_PopMin(t *testing.T) {
+	var set = Make()
+	set.Add("s1", 1)
+	set.Add("s2", 2)
+	set.Add("s3", 3)
+	set.Add("s4", 4)
+
+	var results = set.PopMin(2)
+	if results[0].Member != "s1" || results[1].Member != "s2" {
+		t.Fail()
+	}
+}