Deduplicate and skip falsy column names for geojson queries

Although Windshaft is already removing duplicates and skipping falsy
columns it's better to provide it with good input.

Closes #476

lib/cartodb/models/mapconfig/adapter/analysis-mapconfig-adapter.js

@@ -101,10 +101,7 @@ AnalysisMapConfigAdapter.prototype.getMapConfig = function(user, requestMapConfi
                         analysisSql = sqlQueryWrap.replace(/<%=\s*sql\s*%>/g, analysisSql);
                     }
                     layer.options.sql = analysisSql;
-                    var layerDataviews = getLayerDataviews(layer, dataviews);
-                    layer.options.columns = layerDataviews.reduce(function(columns, dataview) {
-                        return columns.concat(getDataviewColumns(dataview));
-                    }, []);
+                    layer.options.columns = getDataviewsColumns(getLayerDataviews(layer, dataviews));
                 } else {
                     missingNodesErrors.push(
                         new Error('Missing analysis node.id="' + layerSourceId +'" for layer='+layerIndex)
@@ -208,11 +205,22 @@ function getLayerDataviews(layer, dataviews) {
     return layerDataviews;
 }
 
+function getDataviewsColumns(dataviews) {
+    return Object.keys(dataviews.reduce(function(columnsDict, dataview) {
+        getDataviewColumns(dataview).forEach(function(columnName) {
+            if (!!columnName) {
+                columnsDict[columnName] = true;
+            }
+        });
+        return columnsDict;
+    }, {}));
+}
+
 function getDataviewColumns(dataview) {
     var columns = [];
     var options = dataview.options;
     ['column', 'aggregationColumn'].forEach(function(opt) {
-        if (options.hasOwnProperty(opt)) {
+        if (options.hasOwnProperty(opt) && !!options[opt]) {
             columns.push(options[opt]);
         }
     });