connectgaps in scattergl only works when mode is explicitly set to 'lines'

[cpsievert]

Couldn't find this listed in #130, perhaps it should go there.

http://codepen.io/cpsievert/pen/YWPKvz

[etpinard]

scattergl connectgaps was added in #449

There's a bug in it though: it only worked when mode is explicitly set to lines (see here). Thanks for reporting.

[etpinard]

done in #589