Fix for Rails 6 Mime lookups

After Rails 5.2,  controller content_type contained the default charset
causing Mime::LOOKUP not to work.   This was addressed by adding a new
method called `media_type` which would allow Mime lookups to work.

Author: robzolkos

lib/action_controller/caching/pages.rb

@@ -275,7 +275,13 @@ def cache_page(content = nil, options = nil, gzip = Zlib::BEST_COMPRESSION)
               request.path
             end
 
-          if (type = Mime::LOOKUP[self.content_type]) && (type_symbol = type.symbol).present?
+          type = if self.respond_to?(:media_type)
+                   Mime::LOOKUP[self.media_type]
+                 else
+                   Mime::LOOKUP[self.content_type]
+                 end
+
+          if type && (type_symbol = type.symbol).present?
             extension = ".#{type_symbol}"
           end