Use `#set_content_type` instead of `#set_header`

By this commit, `#set_header` is called only via
`#set_content_type`.  This commit makes the role of
`#charset=` more clear.

actionpack/lib/action_dispatch/http/response.rb

@@ -259,7 +259,7 @@ def sending_file=(v)
     def charset=(charset)
       header_info = parsed_content_type_header
       if false == charset
-        set_header CONTENT_TYPE, header_info.mime_type
+        set_content_type header_info.mime_type, nil
       else
         content_type = header_info.mime_type
         set_content_type content_type, charset || self.class.default_charset