FIX: corner detection robust to constant images

Some corner detectors contain a division which runs the risk of
dividing by zero when an image is constant (and so has no gradient
in x or y). Adding a small eps (default 1e-6) to these division
instances forces a finite result to be produced without noticeably
affecting the response in other cases.
FIX: updated unit test

Instead of just checking for NaN, check for NaN and +- Inf,
all of which are responses that we want to avoid from a corner
detector.