imap continuations must have a space after the "+"

prevented at least the gmail/mail (?) android app from appending a sent message
to the sent mailbox.

imapserver/authenticate_test.go

@@ -54,7 +54,7 @@ func TestAuthenticatePlain(t *testing.T) {
 	tc.readstatus("bad")
 
 	tc.cmdf("", "authenticate plain")
-	tc.readprefixline("+")
+	tc.readprefixline("+ ")
 	tc.writelinef("%s", base64.StdEncoding.EncodeToString([]byte("\u0000mjl@mox.example\u0000testtest")))
 	tc.readstatus("ok")
 }

imapserver/idle_test.go

@@ -23,7 +23,7 @@ func TestIdle(t *testing.T) {
 	// todo: test with delivery through smtp
 
 	tc2.cmdf("", "idle")
-	tc2.readprefixline("+")
+	tc2.readprefixline("+ ")
 	done := make(chan error)
 	go func() {
 		defer func() {

imapserver/server.go

@@ -566,7 +566,7 @@ func (c *conn) readCommand(tag *string) (cmd string, p *parser) {
 
 func (c *conn) xreadliteral(size int64, sync bool) string {
 	if sync {
-		c.writelinef("+")
+		c.writelinef("+ ")
 	}
 	buf := make([]byte, size)
 	if size > 0 {
@@ -2541,7 +2541,7 @@ func (c *conn) cmdAppend(tag, cmd string, p *parser) {
 		c.xmailbox(tx, name, "TRYCREATE")
 	})
 	if sync {
-		c.writelinef("+")
+		c.writelinef("+ ")
 	}
 
 	// Read the message into a temporary file.

imapserver/server_test.go

@@ -512,7 +512,7 @@ func TestScenario(t *testing.T) {
 	tc.transactf("ok", "append inbox (\\seen) {%d+}\r\n%s", len(exampleMsg), exampleMsg)
 	tc.transactf("no", "append bogus () {%d}", len(exampleMsg))
 	tc.cmdf("", "append inbox () {%d}", len(exampleMsg))
-	tc.readprefixline("+")
+	tc.readprefixline("+ ")
 	_, err := tc.conn.Write([]byte(exampleMsg + "\r\n"))
 	tc.check(err, "write message")
 	tc.response("ok")