PHPMailer 6.1.5

This is a maintenance release.

• Reject invalid custom headers that are empty or contain breaks
• Various fixes for DKIM issues, especially when using mail() transport
• Drop the l= length tag from DKIM signatures; it's a mild security risk
• Ensure CRLF is used explicitly when needed, rather than static::$LE
• Add a method for trimming header content consistently
• Some minor tweaks to resolve static analyser complaints
• Check that attachment files are readable both when adding and when sending
• Work around Outlook bug in mishandling MIME preamble
• Danish translation improvements