Fixed DOCTYPE in MCHTMLCleaner

We should save original DOCTYPE of html.

If you try to fetch content of "iTunes Connect status update message". It replaces original "HTML 4.01 Transitional" to "XHTML 1.0 Transitional". It results in bad rendering of content in UIWebView.
I've some messages from Yahoo, that doesn't contain Message-Id. I would like to know if a message have Message-Id or not. I use Message-Id to identify messages.