PHPMailer 6.1.7

This is a maintenance release.

• Split SMTP connection into two separate methods
• Undo BC break in PHP vesions 5.2.3 - 7.0.0 introduced in 6.1.2 when injecting callables for address validation and HTML to text conversion
• Save response to SMTP welcome banner as other responses are saved
• Retry stream_select if interrupted by a signal