datastore: datastore.key -> dataset.key doc update.

RE: 3f742eaf8946e0c42a40f148fc19ae39fa88265f missed updating the
jsdoc example code.
tests: pubsub: fix racy tests.
Reorder util.extend logic
Merge pull request #152 from ryanseys/fix-extend

Reorder util.extend logic, fixing dead code
Merge pull request #150 from stephenplusplus/test-pubsub

tests: pubsub: fix racy tests.