Format From, To, Cc & Bcc for RFC1342

[Fank]

EMails needed to be formated like this:

   From: =?US-ASCII?Q?Keith_Moore?= <[email protected]>
   To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= <[email protected]>
   CC: =?ISO-8859-1?Q?Andr=E9_?= Pirard <[email protected]>

Right now the complete content of the field will be encoded which results in

   From: =?US-ASCII?Q?Keith_Moore <[email protected]>?=
   To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen <[email protected]>?=
   CC: =?ISO-8859-1?Q?Andr=E9_ Pirard <[email protected]>?= 

which causes following in mail clients:

https://tools.ietf.org/html/rfc1342

[jordan-wright]

Thanks for sending this in! The only thing I'd recommend changing is the delimiter used to separate addresses as noted in the comments.

Additionally, could you please add a test to ensure the correct behavior is happening? Here's one that I came up with that seems to work, but please double-check it:

func TestHeaderEncoding(t *testing.T) {
	cases := []struct {
		field string
		have  string
		want  string
	}{
		{
			field: "From",
			have:  "Needs Encóding <[email protected]>, Only ASCII <[email protected]>",
			want:  "=?UTF-8?q?Needs_Enc=C3=B3ding?= <[email protected]>, Only ASCII <[email protected]>\r\n",
		},
		{
			field: "Subject",
			have:  "Subject with a 🐟",
			want:  "=?UTF-8?q?Subject_with_a_=F0=9F=90=9F?=\r\n",
		},
	}
	buff := &bytes.Buffer{}
	for _, c := range cases {
		header := make(textproto.MIMEHeader)
		header.Add(c.field, c.have)
		buff.Reset()
		headerToBytes(buff, header)
		want := fmt.Sprintf("%s: %s", c.field, c.want)
		got := buff.String()
		if got != want {
			t.Errorf("invalid utf-8 header encoding. \nwant:%#v\ngot: %#v", want, got)
		}
	}
}

[Fank]

Added more tests to ensure everything works as intended

[jordan-wright]

This LGTM. Thanks @Fank!