[JENKINS-17684] Remove Content-Type header when 304 not modified.

I think that JENKINS-17684 problem may be sending content-type header when http response is 304 in winstone.
Looking a capture [1] , css files are interpreted as 'text/html', but should be 'text/css'.

For more information on this pull request, please see my comment on Jira [2].

[1] http://imgur.com/9Y11bqz
[2] https://issues.jenkins-ci.org/browse/JENKINS-17684?focusedCommentId=178966&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-178966