Fixed type for gridOptions.borderWidth and also changed it as optional parameter.

See: https://github.com/flot/flot/blob/master/API.md
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> "borderWidth" is the width of the border around the plot. Set it to
> 0 to disable the border. Set it to an object with "top", "right",
> "bottom" and "left" properties to use different widths.
Merge pull request #2401 from y-asaba/flot_borderwidth_fix

flot: fixed type for gridOptions.borderWidth