message: use html-to-text scraper for html parts

We were dumping the HTML-parts as-is in the Xapian indexer; however,
it's better to remove the html decoration first, and just pass the text.

We use the new built-in html->text scraper for that.

lib/message/mu-mime-object.cc

@@ -535,8 +535,7 @@ MimePart::to_string() const noexcept
 	if (bytes < 0)
 		return Nothing;
 
-	buffer.data()[bytes]='\0';
-	buffer.resize(buflen);
+	buffer.resize(bytes + 1);
 
 	return buffer;
 }