Include a default title_separator (#1701)

The current code prevented any separator being included when site.title_separator was unset.

_includes/seo.html

@@ -4,9 +4,7 @@
 {%- endif -%}
 {%- assign seo_url = seo_url | default: site.github.url -%}
 
-{% if site.title_separator %}
-  {% assign title_separator = site.title_separator | default: '-' | replace: '|', '|' %}
-{% endif %}
+{% assign title_separator = site.title_separator | default: '-' | replace: '|', '|' %}
 
 {%- if page.title -%}
   {%- assign seo_title = page.title | append: " " | append: title_separator | append: " " | append: site.title -%}