add_the_geom_webmercator: use variable for webmercator field name
add_the_geom_webmercator: use variable for webmercator field name

... and properly quote it

Conflicts:
	lib/tasks/db_maintenance.rake

dbirchak/cartodb,CartoDB/cartodb,dbirchak/cartodb,nyimbi/cartodb,raquel-ucl/cartodb,dbirchak/cartodb,raquel-ucl/cartodb,thorncp/cartodb,DigitalCoder/cartodb,raquel-ucl/cartodb,dbirchak/cartodb,raquel-ucl/cartodb,codeandtheory/cartodb,UCL-ShippingGroup/cartodb-1,nyimbi/cartodb,future-analytics/cartodb,nuxcode/cartodb,splashblot/dronedb,UCL-ShippingGroup/cartodb-1,thorncp/cartodb,thorncp/cartodb,CartoDB/cartodb,nyimbi/cartodb,codeandtheory/cartodb,UCL-ShippingGroup/cartodb-1,CartoDB/cartodb,DigitalCoder/cartodb,bloomberg/cartodb,splashblot/dronedb,codeandtheory/cartodb,codeandtheory/cartodb,DigitalCoder/cartodb,splashblot/dronedb,bloomberg/cartodb,nuxcode/cartodb,thorncp/cartodb,future-analytics/cartodb,nuxcode/cartodb,splashblot/dronedb,bloomberg/cartodb,nuxcode/cartodb,splashblot/dronedb,future-analytics/cartodb,nyimbi/cartodb,CartoDB/cartodb,thorncp/cartodb,DigitalCoder/cartodb,future-analytics/cartodb,CartoDB/cartodb,codeandtheory/cartodb,nuxcode/cartodb,DigitalCoder/cartodb,nyimbi/cartodb,bloomberg/cartodb,future-analytics/cartodb,UCL-ShippingGroup/cartodb-1,raquel-ucl/cartodb,UCL-ShippingGroup/cartodb-1,bloomberg/cartodb,dbirchak/cartodb