Dir#pos= dies if negative value provided

In MRI the Dir#pos= method accepts a negative value. It's effectively a no-op if a read has not occurred yet, or returns nil if at least one read has already occurred, which I assume moves the pointer to the end of the stream. The behavior is curious, and I suspect it's driven by the rules of the underlying seekdir() function, though I haven't thoroughly scoured the source, and the man page for seekdir() made no mention of negative values.

JRuby fails with an Java::JavaLang::ArrayIndexOutOfBoundsException.

I've attached the test suite I'm using for berger_spec.