Proposal to fix #3079 (span_near query should accept slop = -1)

This pull request changes the default slop value from -1 to Integer.MIN_VALUE (in SpanNearQueryParser.java).

The bug remains (i.e. you can specify slop = -2147483648, and the SpanNearQueryParser will raise error "span_near must include [slop]"), but unlike -1, I'm not aware of any case where such a slop value is useful.