Fix removeHeader not removing every applicable headers

Zend\Mail\Headers::removeHeader should remove all headers matching field
name instead of just first match.
Merge pull request #5937 from Xerkus/hotfix/zend-mail-headers-removebyname

Fix Zend\Mail\Headers::removeHeader is not removing every header matching header name
Merge branch 'hotfix/5937' into develop

Close #5937