Make sure Frontmatter data is actually cached. Fixes #903

(Can't return a value to early out of a begin…end expression in Ruby)

middleman-core/lib/middleman-core/core_extensions/front_matter.rb

@@ -59,14 +59,16 @@ module Middleman::CoreExtensions
         @cache[p] ||= begin
           file_data, content = frontmatter_and_content(p)
 
-          return [file_data, content] unless @app.files.exists?("#{path}.frontmatter")
-
+          if @app.files.exists?("#{path}.frontmatter")
             external_data, _ = frontmatter_and_content("#{p}.frontmatter")
 
             [
               external_data.deep_merge(file_data),
               content
             ]
+          else
+            [file_data, content]
+          end
         end
       end