Fix dataset test display

[katie-murphy-ck]

Per the README, this is a small enough change not to require the usual procedure. When running a test on a dataset, an internal Ruby reference to the resource is displayed instead of the dataset name. I stole this syntax from code in other library files.

[katie-murphy-ck]

This is related to issue #125

[slevenick]

This change will not fix the issue described in #125

The issue is that the nested object at libraries/google/bigquery/dataset_access.rb needs a similar to_s method, as that is the object that to_s is called on.

I would prefer to programmatically implement the to_s method on MM generated resources & nested objects. I'll look into possible solutions and keep the issue updated.

[skpaterson]

Thanks for contributing @katie-murphy-ck, hopefully #127 from @slevenick resolves this issue but please let us know if there are any problems!