Add UTF-8 Content-Type to the Django formatter

lib/twine/formatters/django.rb

@@ -70,7 +70,7 @@ module Twine
       end
 
       def format_header(lang)
-        "##\n # Django Strings File\n # Generated by Twine #{Twine::VERSION}\n # Language: #{lang}\n"
+        "##\n # Django Strings File\n # Generated by Twine #{Twine::VERSION}\n # Language: #{lang}\nmsgid \"\"\nmsgstr \"\"\n\"Content-Type: text/plain; charset=UTF-8\\n\""
       end
 
       def format_section_header(section)

test/fixtures/formatter_django.po

@@ -2,7 +2,9 @@
  # Django Strings File
  # Generated by Twine <%= Twine::VERSION %>
  # Language: en
-
+msgid ""
+msgstr ""
+"Content-Type: text/plain; charset=UTF-8\n"
 
 #--------- Section 1 ---------#