Allow value of 0 in L.DivIcon's html parameter

Replacement pull request for https://github.com/Leaflet/Leaflet/pull/1627.
* Fixed white space
* Own branch
* jake test passes

Not sure how to verify that it will pass in Travis, but the "whitespace after :" issue is fixed.  Sorry about the bad pull request before. 


This pull request allows
new L.DivIcon({className:'numbericon',html:0});
to create an icon with a '0' in the label.

Icons created with an html value of 0 ended up with an empty string for html instead of a 0 since options.html evaluated to false.

This sets a default value of false, and checks if options.html !== false.