[mail] Remove unnecessary case insensitive

lowercase and uppercase characters are already included in the range
[mail] Fix assignment to protected methods
[mail] Unify criteria for split fieldname and fieldvalue in the header line.

Also adds @throws tag to interface method docblock.
Merge branch 'hotfix/many-mail-issues' of github.com:Maks3w/zf2 into Maks3w-hotfix/many-mail-issues
Merge branch 'Maks3w-hotfix/many-mail-issues'

Close #5291