Use split strategy for head/tails turbo-carto quantification

lib/cartodb/utils/style/postgres-datasource.js

@@ -23,6 +23,10 @@ var methodTemplates = Object.keys(methods).reduce(function(methodTemplates, meth
     return methodTemplates;
 }, {});
 
+var method2strategy = {
+    headtails: 'split'
+};
+
 function PostgresDatasource (pgQueryRunner, username, query) {
     this.pgQueryRunner = pgQueryRunner;
     this.username = username;
@@ -48,7 +52,7 @@ PostgresDatasource.prototype.getRamp = function (column, buckets, method, callba
             return a - b;
         });
 
-        return callback(null, ramp);
+        return callback(null, { ramp: ramp, strategy: method2strategy[methodName] });
     });
 };