imapserver, imapclient: use lowercase for body encoding

The ABNF uses upper-case in body-fld-enc, however Dovecot uses
lower-case and Outlook for iOS breaks with upper-case encodings.

Author: mercata

imapclient/fetch.go

@@ -975,9 +975,9 @@ func readBodyType1part(dec *imapwire.Decoder, typ string, options *Options) (*im
 	}
 
 	// Content-Transfer-Encoding should always be set, but some non-standard
-	// servers leave it NIL. Default to 7BIT.
+	// servers leave it NIL. Default to 7bit.
 	if bs.Encoding == "" {
-		bs.Encoding = "7BIT"
+		bs.Encoding = "7bit"
 	}
 
 	// TODO: handle errors

imapserver/fetch.go

@@ -615,9 +615,10 @@ func writeBodyType1part(enc *imapwire.Encoder, bs *imap.BodyStructureSinglePart,
 	writeNString(enc, bs.Description)
 	enc.SP()
 	if bs.Encoding == "" {
-		enc.String("7BIT")
+		enc.String("7bit")
 	} else {
-		enc.String(strings.ToUpper(bs.Encoding))
+		// Outlook for iOS chokes on upper-case encodings
+		enc.String(strings.ToLower(bs.Encoding))
 	}
 	enc.SP().Number(bs.Size)