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.

src/sql/tables.sql

@@ -72,6 +72,10 @@ CREATE TABLE msgs (
     timestamp_sent INTEGER DEFAULT 0,
     timestamp_rcvd INTEGER DEFAULT 0,
     hidden INTEGER DEFAULT 0,
+    -- mime_headers column actually contains BLOBs, i.e. it may
+    -- contain non-UTF8 MIME messages.  TEXT was a bad choice, but
+    -- thanks to SQLite 3 being dynamically typed, there is no need to
+    -- change column type.
     mime_headers TEXT,
     mime_in_reply_to TEXT,
     mime_references TEXT,