`union` comparison functions cause assertion failures in VC++ std lib

Running the following query on a Debug-mode built on Windows causes assertion failures:

``` js
r.expr([{a: 1, b: 2}]).union([{a: 1, b: 2}], {interleave: r.desc("a")})
```

The failed assertion is: `Expression: invalid comparator`, and it appears to be coming from `std::priority_queue`.

On Linux, there doesn't seem to be such an assertion, and the query (seemingly) gives a correct result.
