Store mime_headers as BLOBs

Raw MIME messages may contain non-ASCII characters. Attempting to
store them as TEXT by using String::from_utf8_lossy results in
non-ASCII characters being replaced with Unicode U+FFFD "REPLACEMENT
CHARACTER" which is later incorrectly decoded when attempting to parse
`mime_headers` content into HTML.

test-data/message/cp1252-html.eml

@@ -0,0 +1,18 @@
+Subject: test non-utf8 and dc_get_msg_html()
+To: tunis4 <tunis4@testrun.org>
+From: "B. Petersen" <bpetersen@b44t.com>
+Message-ID: <00007126-1efa-290b-2120-200251f50f23@b44t.com>
+Date: Thu, 27 May 2021 17:33:16 +0200
+MIME-Version: 1.0
+Content-Type: text/plain; charset=windows-1252; format=flowed
+Content-Language: en-US
+Content-Transfer-Encoding: 8bit
+
+foo bar <20> <20> <20> <20>
+
+
+-------- Forwarded Message --------
+Subject: 	Foo
+Date: 	Fri, 21 May 2021 08:42:20 +0000
+
+just to force a "more button"