Fix connection URL generation for 'stringtype' parameter in BaseDataSour...

...ce

Since commit e55ef21 introduced support for configuring the "stringtype" connection parameter also for DataSource instances, the resulting connection URL contained a type-o ("=" was missing betwwen parameter and value), resulting in the configured parameter not bein effective at all.

I've also included a test to verify the URL contains what it should contain (without involving a real connection though).