Change type of date/time fields from 'string' to 'DateTimeOffset'

[ggrossman]

Hi, this is my first PR to your repo, let me know if there are things you'd like done differently.

The Zendesk API v2 requires that most date/times be formatted as ISO8601. In the ZendeskApi_v2 library, the types of date/time fields is always string.

It would be helpful for it to be DateTime or DateTimeOffset so that developers don't pass incorrect date/time formats to the Zendesk API. For instance, there have been Zendesk API developers who have passed strings like Friday, September 9, 2014 8:03 AM for due_at to the Zendesk API.

It also is helpful for developers when receiving values back from the Zendesk API; if they're already of type DateTime or DateTimeOffset in the returned result, the developer doesn't have to worry about parsing the result.

I chose DateTimeOffset over DateTime since it includes timezone information and is the more modern choice in the .NET Framework. (Does this make sense?)

This is probably a breaking change so I suppose it'll have to be carefully considered. However, I think most developers would prefer to work with a real date/time object instead of a string in this case.

• Changed date/time types from string to DateTimeOffset?
• Added unit test for creating a ticket with DueAt setting
• Add check for CreatedAt, UpdatedAt to ticket creation test

[eneifert]

I have pulled this down and will release it at the end of October.