#14 Fix Date header decoding on leap years

[Benoît Vallée]

There is an error in msg_date_d: 1 day is missing for the month of March on leap years.
On leap years, we need to add 1 day from March, not from April! ("mon > 1" instead of "mon > 2" because month count start at 0)

Here is the patch.

[Benoît Vallée]