Fixed header calculation not accounting for spacing

[mmundy1]

I was seeing some odd results in terms of the fixed column being under-width. After a bit of trace-back, I found that the difference between my code and the examples was the examples were setting cellspacing to 0. The outerwidth() in the calculation does not account for this, since it is between cell elements
I modded the code, starting with https://github.com/yidas/jquery-freeze-table/blob/master/dist/js/freeze-table.js#L441 to add 2 to the addWidth and then 2 more after the loop; 2px being the default (https://www.w3schools.com/cssref/pr_border-spacing.asp).
But of course that is not a general solution (it needs to look at cell spacing then if undefined, border spacing). I hope to work up a proper patch to submit, but in case I don't, I wanted to file the issue (and on the off chance there was already an option I just missed!)