fix: get value from site config incorrectly

When the value is stored in a dictionary and the value isn't
presented in front matter, the get_value function can not get
value from the site config instead.

_includes/functions/get_value.html

@@ -2,31 +2,41 @@
   {%- assign name = include.params.name -%}
 {%- endif -%}
 
+{%- assign return = nil -%}
+
 {%- assign keys = name | split:'.' -%}
 {%- assign name = keys.first -%}
-
-{%- if page[name] != nil -%}
-  {%- assign return = page[name] -%}
-{%- elsif site[name] != nil -%}
-  {%- assign return = site[name] -%}
-{%- elsif site.data[name] != nil -%}
-  {%- assign return = site.data[name] -%}
-{%- elsif site.defaults[page.layout][name] != nil -%}
-  {%- assign return = site.defaults[page.layout][name] -%}
-{%- elsif site.data.defaults[page.layout][name] != nil -%}
-  {%- assign return = site.data.defaults[page.layout][name] -%}
-{%- elsif layout[name] != nil -%}
-  {%- assign return = layout[name] -%}
-{%- else -%}
-  {%- assign return = include.params.default -%}
-{%- endif -%}
-
 {%- assign keys = keys | shift -%}
-{%- for key in keys -%}
-  {%- assign return = return[key] -%}
-  {%- if return == nil -%}
-    {%- assign return = include.params.default -%}
+
+{%- for step in (1..7) -%}
+
+  {%- case step -%}
+    {%- when 1 -%}
+      {%- assign return = page[name] -%}
+    {%- when 2 -%}
+      {%- assign return = site[name] -%}
+    {%- when 3 -%}
+      {%- assign return = site.data[name] -%}
+    {%- when 4 -%}
+      {%- assign return = site.defaults[page.layout][name] -%}
+    {%- when 5 -%}
+      {%- assign return = site.data.defaults[page.layout][name] -%}
+    {%- when 6 -%}
+      {%- assign return = layout[name] -%}
+    {%- else -%}
+      {%- assign return = include.params.default -%}
+  {%- endcase -%}
+
+  {%- for key in keys -%}
+    {%- assign return = return[key] -%}
+    {%- if return == nil -%}
+      {%- break -%}
+    {%- endif -%}
+  {%- endfor -%}
+
+  {%- if return != nil -%}
     {%- break -%}
   {%- endif -%}
+
 {%- endfor -%}