Fix | Allow writing DateOnly instance as a Variant parameter

[edwardneal]

Description

This resolves an issue described in #3953: when passing a DateOnly instance as a parameter with a SqlDbType of Variant, the current response is an exception with message:

The incoming tabular data stream (TDS) protocol stream is incorrect. The stream ended unexpectedly.

This exception is legitimate, and it's a result of WriteSqlVariantValue and WriteSqlVariantDataRowValue not containing a code path which writes the data - so we're only sending the sql_variant header. Adding these code paths resolves the issue and eliminates the exception.

Issues

Fixes #3953.

Testing

The existing DateTimeVariantTests have been modified to add test coverage.

[ErikEJ]

@edwardneal Would this also fix #3934 ?

[edwardneal]

While it's definitely a good idea to retest once CI has run, I don't know if it'll fix that specific issue. I wasn't able to reproduce it in the first instance, and only bumped into this while testing other scenarios adjacent to it. There are a pair of currently-unreported bugs which crop up when sending DateOnly instances as values in a sql_variant TVP field, and if that code path is being exercised then the #3934 will need a deeper fix.