extend header encoding to cover control characters

[tisdall]

Mail.build() 
|> Mail.put_from("actualfrom@example.com") 
|> Mail.put_subject("some subject\r\nReply-To: cleverhacker@example.com") 
|> Mail.render() 
|> IO.puts()

outputs the following:

Subject: some subject
Reply-To: cleverhacker@example.com
From: actualfrom@example.com

With this PR applied:

Subject: =?UTF-8?Q?some subject=0D=0AReply-To: cleverhacker@example.com?=
From: actualfrom@example.com

Changes proposed in this pull request

Need to encode more than just non-ASCII as control characters can change how a header is read. People shouldn't be putting in control characters, but it's safer to just encode any that may occur.

This only recently became a problem due to #214 . Before that, the comparison between encoded and non-encoded would have been false and it would have used the encoded version.

[bcardarella]

@andrewtimberlake this is 👍 from me.

[andrewtimberlake]

Looks good. Thanks