class ActionDispatch::Response

def charset=(charset)

response.charset = nil # => 'utf-8'
response.charset = 'utf-16' # => 'utf-16'

it sets the charset to +default_charset+.
Sets the HTTP character set. In case of +nil+ parameter 
def charset=(charset)
  content_type = parsed_content_type_header.mime_type
  if false == charset
    set_content_type content_type, nil
  else
    set_content_type content_type, charset || self.class.default_charset
  end
end