Add __str__ method to DatasetReference #1289

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Is your feature request related to a problem? Please describe.
I can str(TableReference) - which is great! I want to also be able to do that with DatasetReference. Easy enough fix.
Describe the solution you'd like
A __str__ method for DatasetReference
Describe alternatives you've considered
This would be the most idiomatic implementation, consistent with DatasetReference
Additional context
Here's a draft PR: #1288