fixed title for og preview, otherwise its "blank" (alshedivat#531)

* fixed title for og preview, otherwise its "blank"

* Update _includes/metadata.html

Co-authored-by: Maruan <alshedivat@users.noreply.github.com>

Author: Ali Kuwajerwala

_includes/metadata.html

@@ -33,9 +33,9 @@
 {%- if site.serve_og_meta %}
 
     <!-- OpenGraph -->
-    <meta property="og:site_name" content="{{ site.title }}" />
+    <meta property="og:site_name" content="{{ title }}" />
     <meta property="og:type" content="website" />
-    <meta property="og:title" content="{%- if page.title -%}{{ site.title }} | {{ page.title }}{%- else -%}{{ site.title }}{%- endif -%}" />
+    <meta property="og:title" content="{%- if page.title -%}{{ title }} | {{ page.title }}{%- else -%}{{ title }}{%- endif -%}" />
     <meta property="og:url" content="{{ page.url | prepend: site.baseurl | prepend: site.url | remove_first: 'index.html' }}" />
     <meta property="og:description" content="{%- if page.description -%}{{ page.description }}{%- else -%}{{ site.description }}{%- endif -%}" />
     {% if page.og_image or site.og_image -%}
@@ -45,7 +45,7 @@
 
     <!-- Twitter card -->
     <meta name="twitter:card" content="summary" />
-    <meta name="twitter:title" content="{%- if page.title -%}{{ page.title }}{%- else -%}{{ site.title }}{%- endif -%}" />
+    <meta name="twitter:title" content="{%- if page.title -%}{{ page.title }}{%- else -%}{{ title }}{%- endif -%}" />
     <meta name="twitter:description" content="{%- if page.description -%}{{ page.description }}{%- else -%}{{ site.description }}{%- endif -%}" />
     {% if page.og_image or site.og_image -%}
     <meta name="twitter:image" content="{%- if page.og_image -%}{{ page.og_image }}{%- else -%}{{ site.og_image }}{%- endif -%}" />