provide more helpful error for negative duration (#49)

conn.go

@@ -73,6 +73,13 @@ func (c *Conn) Close() error {
 }
 
 func (c *Conn) cmd(t *Tube, ts *TubeSet, body []byte, op string, args ...interface{}) (req, error) {
+	// negative dur checking
+	for _, arg := range args {
+		if d, _ := arg.(dur); d < 0 {
+			return req{}, fmt.Errorf("duration must be non-negative, got %v", time.Duration(d))
+		}
+	}
+
 	r := req{c.c.Next(), op}
 	c.c.StartRequest(r.id)
 	defer c.c.EndRequest(r.id)

conn_test.go

@@ -44,6 +44,16 @@ func TestNameBadChar(t *testing.T) {
 	}
 }
 
+func TestNegativeDuration(t *testing.T) {
+	c := NewConn(mock("", ""))
+	tube := NewTube(c, "foo")
+	for _, d := range []time.Duration{-100 * time.Millisecond, -2 * time.Second} {
+		if _, err := tube.Put([]byte("hello"), 0, d, d); err == nil {
+			t.Fatalf("put job with negative duration %v expected error, got nil", d)
+		}
+	}
+}
+
 func TestDeleteMissing(t *testing.T) {
 	c := NewConn(mock("delete 1\r\n", "NOT_FOUND\r\n"))