Fix 1904-based date handling

Fixes conversion of date values in Excel cells that use 1904-based date handling. Excel primarily uses dates based on a serial number since Jan 1 1900, with an adjustment to account for a bug in 1900 leap year calculation.  Mac Excel versions use dates based on Jan 1 1904. RubyXL was using the 1900 adjustment calculation for the 1904 dates, resulting in a two day offset from the correct value for 1904-based dates. This p.r. fixes that bug.