compose: parse headers correctly from -H

By using :compose -H <header> a user should be able to add
arbitrary headers to an email.  The existing implementation from
5b523880b4 added the headers as lines
to the beginning of the body.  These lines were not interpreted as
headers anywhere and ended up as plain text in the body of the email.
Fix the code to parse and add the headers correctly.

Signed-off-by: Evan Gates <evan.gates@gmail.com>
Tested-by: Moritz Poldrack <moritz@poldrack.dev>

commands/account/compose.go

@@ -2,9 +2,14 @@ package account
 
 import (
 	"errors"
+	"fmt"
+	"io"
+	gomail "net/mail"
 	"regexp"
 	"strings"
 
+	"github.com/emersion/go-message/mail"
+
 	"git.sr.ht/~rjarry/aerc/logging"
 	"git.sr.ht/~rjarry/aerc/models"
 	"git.sr.ht/~rjarry/aerc/widgets"
@@ -38,9 +43,17 @@ func (Compose) Execute(aerc *widgets.Aerc, args []string) error {
 		template = aerc.Config().Templates.NewMessage
 	}
 
+	msg, err := gomail.ReadMessage(strings.NewReader(body))
+	if errors.Is(err, io.EOF) { // completely empty
+		msg = &gomail.Message{Body: strings.NewReader("")}
+	} else if err != nil {
+		return fmt.Errorf("mail.ReadMessage: %w", err)
+	}
+	headers := mail.HeaderFromMap(msg.Header)
+
 	composer, err := widgets.NewComposer(aerc, acct,
 		aerc.Config(), acct.AccountConfig(), acct.Worker(),
-		template, nil, models.OriginalMail{})
+		template, &headers, models.OriginalMail{})
 	if err != nil {
 		return err
 	}
@@ -56,7 +69,7 @@ func (Compose) Execute(aerc *widgets.Aerc, args []string) error {
 	go func() {
 		defer logging.PanicHandler()
 
-		composer.AppendContents(strings.NewReader(body))
+		composer.AppendContents(msg.Body)
 	}()
 	return nil
 }