fix sankey attribute to pass test-image

Author: antoinerg

src/traces/sankey/attributes.js

@@ -14,6 +14,7 @@ var colorAttrs = require('../../components/color/attributes');
 var fxAttrs = require('../../components/fx/attributes');
 var domainAttrs = require('../../plots/domain').attributes;
 
+var extendFlat = require('../../lib/extend').extendFlat;
 var overrideAll = require('../../plot_api/edit_types').overrideAll;
 
 module.exports = overrideAll({
@@ -121,7 +122,7 @@ module.exports = overrideAll({
             role: 'style',
             description: 'Sets the thickness (in px) of the `nodes`.'
         },
-        hoverinfo: Object.assign({}, plotAttrs.hoverinfo, {flags: []}),
+        hoverinfo: extendFlat({}, plotAttrs.hoverinfo, {flags: []}),
         hoverlabel: fxAttrs.hoverlabel, // needs editType override,
         description: 'The nodes of the Sankey plot.'
     },
@@ -181,7 +182,7 @@ module.exports = overrideAll({
             role: 'info',
             description: 'A numeric value representing the flow volume value.'
         },
-        hoverinfo: Object.assign({}, plotAttrs.hoverinfo, {flags: []}),
+        hoverinfo: extendFlat({}, plotAttrs.hoverinfo, {flags: []}),
         hoverlabel: fxAttrs.hoverlabel, // needs editType override,
         description: 'The links of the Sankey plot.'
     }