Support a default value for DbString.IsAnsi

It is painful and easy to forget to set `DbString.IsAnsi = true` when
dealing with databases that mainly use char/varchar data types (as
opposed to nchar/nvarchar).

This patch allows a default value for `DbString.IsAnsi` to be configured
via the `DefaultStringDbType` static property.
Commit DbString.DefaultStringDbType test
Fixed typo

'exceptionThrow' -> 'exceptionThrown'
Merge remote-tracking branch 'kppullin/defaultStringType'
Change default for strings to AnsiString