Add explicit goDeeper option to wraped program;Fixes #1686

According to the commit message of 279e038:

" [...] Helper authors now need to take care to return the same context
value whenever it is conceptually the same and to avoid behaviors that
may execute children under the current context in some situations and
under different contexts under other situations."

I belive that metric is too implicit, as the issue demonstrates. This
is because in this case the context value is not conceptually the same,
but it is de facto the same value.

Instead this suggests to use an explicit option flag "goDeeper" to mark
when a helper definitely should go deeper. This also should be backwards
safe, as it doesn't change the behavior of existing helpers, builtin or
custom, except the builtin "each",

lib/handlebars/helpers/each.js

@@ -52,7 +52,8 @@ export default function(instance) {
           blockParams: blockParams(
             [context[field], field],
             [contextPath + field, null]
-          )
+          ),
+          goDeeper: true
         });
     }
 

lib/handlebars/runtime.js

@@ -323,9 +323,10 @@ export function wrapProgram(
   function prog(context, options = {}) {
     let currentDepths = depths;
     if (
-      depths &&
-      context != depths[0] &&
-      !(context === container.nullContext && depths[0] === null)
+      (depths &&
+        context != depths[0] &&
+        !(context === container.nullContext && depths[0] === null)) ||
+      options.goDeeper
     ) {
       currentDepths = [context].concat(depths);
     }