Fix frontmatter parsing changes from #1608

Only allow frontmatter at the top of the page:
  Now manually removing encoding lines as before

Fix handling of postscript data:
  If there is no frontmatter and the filetype is known,
  parse the whole file

Allow empty frontmatter

Update the documentation comments

middleman-core/lib/middleman-core/util/data.rb

@@ -23,19 +23,22 @@ module Middleman::Util::Data
       return [{}, nil]
     end
 
+    /
+      \A(.*?coding:.*?\n)?
+      (?<start>[-;]{3})[ ]*\r?\n
+      (?<frontmatter>.*?)[ ]*\r?\n?
+      (?<stop>[-.;]{3})[ ]*\r?\n?
+      (?<additional_content>.*)
+    /mx =~ content
+
+    unless frontmatter
       case known_type
       when :yaml
         return [parse_yaml(content, full_path), nil]
       when :json
         return [parse_json(content, full_path), nil]
       end
-
-    /
-      (?<start>^[-;]{3})[ ]*\r?\n
-      (?<frontmatter>.*?)[ ]*\r?\n
-      (?<stop>^[-.;]{3})[ ]*\r?\n?
-      (?<additional_content>.*)
-    /mx =~ content
+    end
 
     case [start, stop]
     when %w[--- ---], %w[--- ...]
@@ -49,10 +52,10 @@ module Middleman::Util::Data
 
   # Parse YAML frontmatter out of a string
   # @param [String] content
-  # @return [Array<Hash, String>]
+  # @return [Hash]
   Contract String, Pathname, Bool => Hash
   def parse_yaml(content, full_path)
-    symbolize_recursive(YAML.load(content))
+    symbolize_recursive(YAML.load(content) || {})
   rescue StandardError, Psych::SyntaxError => error
     warn "YAML Exception parsing #{full_path}: #{error.message}"
     {}
@@ -60,10 +63,10 @@ module Middleman::Util::Data
 
   # Parse JSON frontmatter out of a string
   # @param [String] content
-  # @return [Array<Hash, String>]
+  # @return [Hash]
   Contract String, Pathname => Hash
   def parse_json(content, full_path)
-    symbolize_recursive(JSON.parse(content))
+    symbolize_recursive(JSON.parse(content) || {})
   rescue StandardError => error
     warn "JSON Exception parsing #{full_path}: #{error.message}"
     {}