Duplicate headers due to case-sensitive checking

Problem/Motivation

Email headers are case-insensitive but this module only checks for a specific case. This can result in duplicate headers such as Bcc, bcc, Reply-to, reply-to, Reply-To etc.

Steps to reproduce

Add the same headers with different case.

Proposed resolution

Convert all headers to lowercase before checking for them. PHPMailer has specific methods for adding headers so case doesn't matter.