Add test for pool's TestOnBorrow locking.

When I read `Pool.Get()` earlier today, I thought there was a locking bug because it had lock/unlock inside a for loop. I wrote this test to verify that the bug exists before fixing, but I turned out to be mistaken. Your Pool locking is solid. But I thought you might benefit from this test I wrote, so you don't have any regressions in the future.
Merge pull request #93 from jart/patch-1

Add test for pool's TestOnBorrow locking.