Handle duplicate headers sent from a client

When a client sends duplicate headers to the FTW Web server in logstash/kibana, the process dies with the following error:

```
/opt/logstash/vendor/bundle/jruby/1.9/gems/ftw-0.0.39/lib/rack/handler/ftw.rb:107:in `run'
NameError: undefined local variable or method `v' for #<FTW::HTTP::Headers:0x5b9dd081>
               each at /opt/logstash/vendor/bundle/jruby/1.9/gems/ftw-0.0.39/lib/ftw/http/headers.rb:120
                map at org/jruby/RubyArray.java:2409
               each at /opt/logstash/vendor/bundle/jruby/1.9/gems/ftw-0.0.39/lib/ftw/http/headers.rb:120
               each at org/jruby/RubyHash.java:1339
               each at /opt/logstash/vendor/bundle/jruby/1.9/gems/ftw-0.0.39/lib/ftw/http/headers.rb:118
     handle_request at /opt/logstash/vendor/bundle/jruby/1.9/gems/ftw-0.0.39/lib/rack/handler/ftw.rb:178
  handle_connection at /opt/logstash/vendor/bundle/jruby/1.9/gems/ftw-0.0.39/lib/rack/handler/ftw.rb:136
  handle_connection at /opt/logstash/lib/logstash/kibana.rb:12
                run at /opt/logstash/vendor/bundle/jruby/1.9/gems/ftw-0.0.39/lib/rack/handler/ftw.rb:10
```
This pull request fixes this issue.