ReduceFnRunner fetches side input from the wrong window

This https://github.com/apache/incubator-beam/blame/master/runners/core-java/src/main/java/org/apache/beam/runners/core/ReduceFnContextFactory.java#L529 is incorrect - I broke it in https://github.com/apache/incubator-beam/commit/90a0d0e13fa0332df805b79b1dc64860d9590217#diff-16edced77586e39a5f31907f4ced51b5R530

It uses windowing strategy of the main input to do .getSideInputWindow() instead of windowing strategy of the side input.

The fix is very simple; trying to come up with a test...