Bugfix/duplicat subject header

[weierophinney]

This fixes the issue described in zendframework/zend-mail#22

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Originally posted by @arueckauer at zendframework/zend-mail#225

[glensc]

@weierophinney can you please edit the issue body to fix link, it must be https://github.com/zendframework/zend-mail/issues/22 not #22 in this project.