simplify intersectBitmapRun output to remove a conversion
simplify intersectBitmapRun output to remove a conversion

If the total number of things returned was small enough to
make an array, intersectBitmapRun converted to an array. This
seems possibly-premature; future processing might well prefer
a bitmap. We know everything gets optimized before being
written out, let's not convert without a specific reason. But
also, let's use an array no matter which container is small
enough to prove that we can do so safely.

Fixes #854.

travisturner/pilosa