Implement MySqlDateTime, AllowZeroDateTime

[bgrainger]

The PeachPie project updated to MySqlConnector (peachpiecompiler/peachpie#209) but then reverted the change because AllowZeroDateTime isn't supported.

If this connection string option were supported, they might be able to adopt the library again. The documentation of the connection string setting is:

If set to True, MySqlDataReader.GetValue() returns a MySqlDateTime object for date or datetime columns that have disallowed values, such as zero datetime values, and a System.DateTime object for valid values. If set to False (the default setting) it causes a System.DateTime object to be returned for all valid values and an exception to be thrown for disallowed values, such as zero datetime values.

Implementing this may also fix #501.

[bgrainger]

PeachPie would also need PipeName #454 and InteractiveSession #510; just implementing AllowZeroDateTime isn't sufficient for their needs.