Inconsistent Capitalization in Header Array Keys among modules causes duplicate header parameters (Reply-to)

I am using the webform module and mandrill module together.

Emails were being bounced by certain recipients with the error:
"smtp;550 Messages should have one or no Reply-To headers, not 2."

The issue was the webmail module was setting a reply-to attribute in the header array using the array key "Reply-to". Mandrill's module is checking for an array key "Reply-To" (Capital T in "To"). If mandrill does not find that key, it adds an array item with the key "Reply-To". This results in two array items: $header['Reply-to'] and $header['Reply-To'] and as a result two Reply-To headers in the email.

I confirmed the email specifications declare the headers case insensitive.
https://stackoverflow.com/a/6143644

Therefore this issue is drupal specific and about the interplay of different contributed modules extending one another.

I searched core and found a mix of uses of "Reply-to", "Reply-To" and "reply-to". Googling around "Reply-to:" seems most popular.

A simple fix could be to confirm to webmail's module and use the key 'Reply-to'. I will provide a patch below.

However, I may suggest a more robust solution that can handle any mixed capitalization. I will provide that patch below also.