Regression: Accept layer option in the static map urls

docs/static_maps_api.md

@@ -11,7 +11,7 @@ Begin by instantiating either a Named or Anonymous Map using the `layergroupid t
 #### Definition
 
 ```bash
-GET /api/v1/map/static/center/{token}/{z}/{lat}/{lng}/{width}/{height}.{format}
+GET /api/v1/map/static/center/{token}/{z}/{lat}/{lng}/{width}/{height}.{format}{{?}extra_options}
 ```
 
 #### Params
@@ -58,6 +58,9 @@ Note: you can see this endpoint as
 GET /api/v1/map/static/bbox/{token}/{west},{south},{east},{north}/{width}/{height}.{format}`
 ```
 
+#### Extra options
+ * Layer: List of layers to be shown in the image (by default `all`), for example `?layer=0,1`.
+
 ### Named Map
 
 #### Definition

lib/cartodb/api/middlewares/map-store-map-config-provider.js

@@ -10,12 +10,13 @@ module.exports = function createMapStoreMapConfigProvider (
     return function createMapStoreMapConfigProviderMiddleware (req, res, next) {
         const { user, token, cache_buster, api_key } = res.locals;
         const { dbuser, dbname, dbpassword, dbhost, dbport } = res.locals;
-        const { layer, z, x, y, scale_factor, format } = req.params;
+        const { layer: layerFromParams, z, x, y, scale_factor, format } = req.params;
+        const { layer: layerFromQuery } = req.query;
 
         const params = {
             user, token, cache_buster, api_key,
             dbuser, dbname, dbpassword, dbhost, dbport,
-            layer, z, x, y, scale_factor, format
+            layer: (layerFromQuery || layerFromParams), z, x, y, scale_factor, format
         };
 
         if (forcedFormat) {

test/acceptance/analysis/named-maps.js

@@ -261,6 +261,27 @@ describe('named-maps analysis', function() {
             );
         });
 
+        it('should fail to retrieve static map preview via layergroup ' +
+           'when filtering by invalid layers', function(done) {
+            assert.response(
+                server,
+                {
+                    url: '/api/v1/map/static/center/' + layergroupid + '/4/42/-3/320/240.png?layer=1',
+                    method: 'GET',
+                    encoding: 'binary',
+                    headers: {
+                        host: username
+                    }
+                },
+                {
+                    status: 400
+                },
+                function(res, err) {
+                    done(err);
+                }
+            );
+        });
+
         it('should return and an error requesting unsupported image format', function(done) {
             assert.response(
                 server,