eve-http: print <unknown> like in eve-files

When UA or Host are unknown, print <unknown> instead of <useragent
unknown> or <hostname unknown>.

Bug #1131. https://redmine.openinfosecfoundation.org/issues/1131

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/227
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/147