Default exempi date values

Exempi sets xmp Modify Date and Create Date tags to 1904:01:01 00:00Z automatically if exempi is used and the corresponding tags are not defined (and if date values are not already set in xmp?).

[XMP]           XMP Toolkit                     : Exempi + XMP Core 6.0.0
[XMP]           Create Date                     : 1904:01:01 00:00Z
[XMP]           Modify Date                     : 1904:01:01 00:00Z
[XMP]           GPS Altitude Ref                : Below Sea Level