dbDataType tests based on dispatch on driver

[imanuelcostigan]

The dbDataType tests are included in the test_driver set rather than in the test_connection set. I can see how this works for RMySQL and RSQLite. However:

1. The DBI vignette seems to suggest to define this method for a Connection. This doesn't preclude the Connection method simply wrapping the Driver method - as is the case for RMySQL and RSQLite.
2. It makes this difficult for some DBI backends where the connection definitively establishes what SQL types are available. For example, the DATE T-SQL type is not available in versions of SQL Server earlier than SQL Server 2008. The driver doesn't know about this, but the connection does (because you can determine the version number associated with SQL Server.

It is possible to skip the test_driver tests and it might be the case that for most SQL backends, the Driver does specify what SQL backend types are available. However, I am wondering if putting these in the Driver section of the test harness is the natural home for this.

[krlmlr]

This is an omission, dbDataType() should also be tested for a connection. (Drivers are free to skip the driver-based tests if the connection is necessary, or perhaps we add a tweak that governs this.) Happy to review a PR.

Note that dbDataType() should return an SQL object (#65), but this isn't tested yet.