Bugfix: calendar -> JD broken for arrays of JD

If a Julian calendar cutoff was specified, arrays of dates were failing
to turn into JDs but raising an exception instead.  See #450.

skyfield/tests/test_timelib.py

@@ -450,11 +450,23 @@ def test_raw_julian_gregorian_cutover():
     assert compute_calendar_date(gregory + 0, gregory) == (1582, 10, 15)
     assert compute_calendar_date(gregory + 1, gregory) == (1582, 10, 16)
 
+    jd = np.arange(gregory - 2, gregory + 2)
+    assert [list(a) for a in compute_calendar_date(jd, gregory)] == [
+        [1582, 1582, 1582, 1582],
+        [10, 10, 10, 10],
+        [3, 4, 15, 16],
+    ]
+
     assert julian_day(1582, 10, 3, gregory) == (gregory - 2)
     assert julian_day(1582, 10, 4, gregory) == (gregory - 1)
     assert julian_day(1582, 10, 15, gregory) == (gregory + 0)
     assert julian_day(1582, 10, 16, gregory) == (gregory + 1)
 
+    days = [3, 4, 15, 16]
+    assert list(julian_day(1582, 10, np.array(days), gregory)) == [
+        2299159, 2299160, 2299161, 2299162,
+    ]
+
 def test_constructor_julian_gregorian_cutover(ts, time_scale_name):
     if sys.version_info <= (3,):
         return  # Python 2 time.strftime() complains about the year 1582

skyfield/timelib.py

@@ -796,9 +796,9 @@ def julian_day(year, month=1, day=1, julian_before=None):
     f = (month + 9) % 12
     e = 1461 * g // 4 + day - 1402
     J = e + (153 * f + 2) // 5
-    if (julian_before is None) or (J >= julian_before):
-        J += 38
-        J -= (g + 184) // 100 * 3 // 4
+
+    mask = 1 if (julian_before is None) else (J >= julian_before)
+    J += (38 - (g + 184) // 100 * 3 // 4) * mask
     return J
 
 def julian_date(year, month=1, day=1, hour=0, minute=0, second=0.0):