[http] Adds docblocks for HeaderInterface interface and methods
[http] Unify criteria for split name and value in header line

Adds a static method in GenericHeader with the responsibility of split both parts.
hotfix 5253
Merge branch 'hotfix/mail-proper-cast-headers-tostring'

Close #5303
Merge branch 'hotfix/http-header-format' of github.com:Maks3w/zf2 into Maks3w-hotfix/http-header-format
Merge branch 'Maks3w-hotfix/http-header-format' into develop

Forward port #5302