Fix rendering Liquid constructs in excerpts (#6970)

Merge pull request 6970

lib/jekyll/excerpt.rb

@@ -9,9 +9,12 @@ module Jekyll
     attr_writer   :output
 
     def_delegators :@doc, :site, :name, :ext, :extname,
-                          :render_with_liquid?, :collection, :related_posts,
+                          :collection, :related_posts,
+                          :coffeescript_file?, :yaml_file?,
                           :url, :next_doc, :previous_doc
 
+    private :coffeescript_file?, :yaml_file?
+
     # Initialize this Excerpt instance.
     #
     # doc - The Document.
@@ -84,6 +87,10 @@ module Jekyll
       false
     end
 
+    def render_with_liquid?
+      !(coffeescript_file? || yaml_file? || !Utils.has_liquid_construct?(content))
+    end
+
     protected
 
     # Internal: Extract excerpt from the content