allowNull: true with no min or max results in Infinity/-Infinity

[mrmcderm]

I noticed some interesting behavior and I was not sure if it was intended/by design or if it was a defect.

If I created a spinner that allowed nulls, had a step value, but did not have a min or a max:

$('#mySpinner').spinner( { increment: 'fast', allowNull: true, step: 0.001 });

then every time I tried to spin, the textbox was populated with either 'Infinity' or '-Infinity' depending on whether I tried to spin up or down.

I noticed that in the _doSpin function, if value == null and the step is greater than 0, then value is set to min or max, but if those aren't set, they return the string '-Infinity' or 'Infinity' respectively.

I added a check after that where if value is '-Infinity' or 'Infinity' then set value to 0.

The resulting behavior is that if I have an empty (null) spinner that has a step value, and I attempt to spin in either direction, it succesfully beings to spin from 0.

So far I haven't seen any issues with this change, but I'm not sure if this is the right way to address the problem.