DBF header date set to 1 Jan 2049 when creating new files

[inkysquid]

When the DbfHeader class is initialised, there is no way to set the _updateDate value, so it ends up as 1 Jan 2049 after the following:

1. _updateDate defaults to DateTime.MinValue (1 Jan 0001)
2. 1 - 1900 = -1899
3. (byte)-1899 = 149
4. When the file is read back: 149 + 1900 = 2049

Ideas

• Consider defaulting _updateDate to the current time (using TimeProvider?)
• Expose the _updateDate with a prop so it can be set explicitly