Fix PropertySet's unique_indexes method.

[J-F-Liu]

Related Issue: #281

[J-F-Liu]

rake run result:

Finished in 58.35322 seconds

12822 examples, 0 failures, 3505 pending

[tpitale]

@J-F-Liu Do you have a failing test case that this fixes?

[J-F-Liu]

@tpitale Sorry, I didn't write a test case for this.

[tpitale]

@J-F-Liu Looks like you had something from #281's original report. If you could add something I'll merge this.

[J-F-Liu]

I write a test case in rspec, but the InMemoryAdapter doesn't have the ability to check uniqueness, so the expected error is not raised.

[tpitale]

I don't think we want to include this change to the API for property unique indexes to support a new case at this time. We're drowning maintenance and adding new cases will only make this worse.