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

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.