Add clearSubject before setSubject

[kassner]

In a very few cases, I'd experienced some Subject set twice exceptions. We can clear the subject before to avoid any exception.

[aschroder]

Thanks for the pull request. I too have seen this error, and I agree that the extension could prevent it by clearing the subject right before setting it just as you have suggested.

However, I do not think this change should be made in SMTP Pro. The parts of the extension where you suggest modification mirror the core Magento functionality intentionally. This is important so that any email that did or did not send in core Magento, will have the same behavior in SMTP Pro.

I'll close this PR but suggest any one facing the subject set twice error coms here for a patch.

[vishalpatel14]

Does it support Multi-Website Magento Email configurations for different servers?