[geary/wip/362-utf8-attachments] Update PreviewText.with_header to use convenience APIs

[Michael Gratton <mjog src gnome org>]

commit addebc0309c3d542d16d06ae18d4ec8f7b8b1455
Author: Michael Gratton <mike vee net>
Date:   Thu Jul 18 15:36:26 2019 +1000

    Update PreviewText.with_header to use convenience APIs

 src/engine/rfc822/rfc822-message-data.vala | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)
---
diff --git a/src/engine/rfc822/rfc822-message-data.vala b/src/engine/rfc822/rfc822-message-data.vala
index f5eeb628..669d34ae 100644
--- a/src/engine/rfc822/rfc822-message-data.vala
+++ b/src/engine/rfc822/rfc822-message-data.vala
@@ -408,20 +408,12 @@ public class Geary.RFC822.PreviewText : Geary.RFC822.Text {
                 );
                 gpart.set_content_object(body);
 
-                ByteArray output = new ByteArray();
-                GMime.StreamMem output_stream =
-                    new GMime.StreamMem.with_byte_array(output);
-                output_stream.set_owner(false);
-
                 try {
-                    part.write_to_stream(
-                        output_stream, Part.EncodingConversion.NONE
+                    Memory.Buffer preview_buffer = part.write_to_buffer(
+                        Part.EncodingConversion.UTF8
                     );
-                    uint8[] data = output.data;
-                    data += (uint8) '\0';
-
                     preview_text = Geary.RFC822.Utils.to_preview_text(
-                        (string) data,
+                        preview_buffer.to_string(),
                         is_html ? TextFormat.HTML : TextFormat.PLAIN
                     );
                 } catch (RFC822Error err) {