REST-406: Add X-Content-Type-Options: nosniff header if and only if there's no such header set yet

Better, alternative solution to this one https://bitbucket.org/atlassian/atlassian-rest/pull-requests/166/rest-406-add-switch-for-applying/diff

Add X-Content-Type-Options: nosniff header once, i.e. if and only if there's no such header set yet, both in ContainerResponse or ServletResponse.

Hence, this change will not introduce security risk, there's no way to turn this off. The additional means are good precaution means, to get the right header value in the end.