Date histogram: Add missing GROUP BY column

lib/cartodb/models/dataview/histograms/date-histogram.js

@@ -56,7 +56,7 @@ __wd_buckets AS
         ${ctx.query}
     ) __source, __wd_tz
     ${condition_str}
-    GROUP BY timestamp
+    GROUP BY timestamp, __wd_tz.name
 ),`;
 }