Invalid Coptic calendar date

[lkarp-744]

Converting to the Coptic calendar from a Julian day results in an invalid month:

Coptic Leap Year (e.g. 1891)

>>> convertdate.coptic.from_jd(2515716.0)
(1891, 13, 5)
>>> convertdate.coptic.from_jd(2515716.5)
(1891, 13, 6)
>>> convertdate.coptic.from_jd(2515717.0)
(1892, 0, 30) # should be (1891, 13, 6)
>>> convertdate.coptic.from_jd(2515717.5)
(1892, 1, 1)
>>> convertdate.coptic.from_jd(2515718.0)
(1892, 1, 1)
>>> 

Coptic Regular Year (e.g. 1889)

>>> convertdate.coptic.from_jd(2514984.5)
(1889, 13, 4)
>>> convertdate.coptic.from_jd(2514985.0)
(1889, 13, 4)
>>> convertdate.coptic.from_jd(2514985.5)
(1889, 13, 5)
>>> convertdate.coptic.from_jd(2514986.0)
(1890, 0, 30) # should be (1890, 13, 5)
>>> convertdate.coptic.from_jd(2514986.5)
(1890, 1, 1)
>>> convertdate.coptic.from_jd(2514987.0)
(1890, 1, 1)
>>>