[permalink] Ensure that the permalink is correctly set #199 (#200)

It ensures that the permalink_lang attribute is set correctly and can be used when a permalink attribute is set at the site level.
It is, indeed, not enough to check if the permalink is undefined. The permalink might be set to empty. Moreover, we should check that the page has a lang attribute isn't empty as well.

lib/jekyll/polyglot/patches/jekyll/site.rb

@@ -138,7 +138,7 @@ def coordinate_documents(docs)
         lang_exclusive = doc.data['lang-exclusive'] || []
         url = doc.url.gsub(regex, '/')
         page_id = doc.data['page_id'] || url
-        doc.data['permalink'] = url unless defined?(doc.data['permalink'])
+        doc.data['permalink'] = url if doc.data['permalink'].to_s.empty? && !doc.data['lang'].to_s.empty?
 
         # skip entirely if nothing to check
         next if @file_langs.nil?
@@ -185,7 +185,7 @@ def assignPageLanguagePermalinks(doc, docs)
       end
     end
 
-    # performs any necesarry operations on the documents before rendering them
+    # performs any necessary operations on the documents before rendering them
     def process_documents(docs)
       # return if @active_lang == @default_lang