Make jsAttributesHandler use jsAttributes

This addresses this comment:

https://github.com/yesodweb/yesod/pull/1622#discussion_r317467498

yesod-core/src/Yesod/Core/Class/Yesod.hs

@@ -252,13 +252,12 @@ class RenderRoute site => Yesod site where
     -- | Same as @jsAttributes@ but allows you to run arbitrary Handler code
     --
     -- This is useful if you need to add a randomised nonce value to the script
-    -- tag generated by @widgetFile@. If you use both this function and
-    -- @jsAttributes@, the two attribute maps will be merged, favouring the
-    -- keys from @jsAttributes@ in case of a collision.
+    -- tag generated by @widgetFile@. If this function is overridden then
+    -- @jsAttributes@ is ignored.
     --
     -- @since 1.6.16
     jsAttributesHandler :: HandlerFor site [(Text, Text)]
-    jsAttributesHandler = pure []
+    jsAttributesHandler = getYesod >>= return . jsAttributes
 
     -- | Create a session backend. Returning 'Nothing' disables
     -- sessions. If you'd like to change the way that the session
@@ -532,7 +531,7 @@ widgetToPageContent :: Yesod site
                     => WidgetFor site ()
                     -> HandlerFor site (PageContent (Route site))
 widgetToPageContent w = do
-  jsAttrsFromHandler <- jsAttributesHandler
+  jsAttrs <- jsAttributesHandler
   HandlerFor $ \hd -> do
   master <- unHandlerFor getYesod hd
   ref <- newIORef mempty
@@ -544,10 +543,6 @@ widgetToPageContent w = do
   let title = maybe mempty unTitle mTitle
       scripts = runUniqueList scripts'
       stylesheets = runUniqueList stylesheets'
-      jsAttrs =
-        let jsAttrsStatic = Map.fromList (jsAttributes master)
-            jsAttrsDynamic = Map.fromList jsAttrsFromHandler
-         in Map.toList $ Map.union jsAttrsStatic jsAttrsDynamic
 
   flip unHandlerFor hd $ do
     render <- getUrlRenderParams