Fix a fatal error when assert WWW-Authenticate header is sent

Fix a fatal error that occured when an assert[Not]ReponseHeader* method is called to check a MultipleHeaderInterface (like WWW-Authenticate).

When dealing with MultipleHeaderInterface :

* assert* methods will check that at least one header match.
* assertNot* methods will check that every headers doesn't match.
Fix a fatal error when assert WWW-Authenticate header is sent

Remove an assertEquals that had no use.
Merge pull request #5659 from lucascorbeaux/hotfix/ZF-5658

Fix a fatal error when assert WWW-Authenticate header is sent - Hotfix/5658
Merge branch 'hotfix/5659'

Close #5659
Fixes #5658