[mail] Fix headers cast to string

Due lazy loading of specific headers is not possible a direct access to the headers attribute.
Instead use the own implementation of ArrayAccess allow lazy load through the get method which do lazy loading of headers.
Merge branch 'hotfix/mail-proper-cast-headers-tostring' of github.com:Maks3w/zf2 into hotfix/mail-proper-cast-headers-tostring
Merge branch 'hotfix/mail-proper-cast-headers-tostring'

Close #5303