APIException with If-Modified-Since too small

[meboz]

relates to this discussion, but Ill raise it here then add some validation in the wrapper.

https://community.xero.com/developer/discussion/56611/

If you supply an If-Modified-Since header that is smaller than 01/01/1753 (the min value for a SqlDateTime), the API call executes but returns 500.

For users of the wrapper library, we could validate this client side and provide some useful exception and msg before the call hits the server.

Validation should probably still occur server side as it took me some time to debug why the server threw a 500.

Ill provide a test and a patch and submit a pull request.

Cheers, Byron