Conditional get is causing exceptions during regular usage

Conditional get is causing exceptions during regular usage, most commonly we see HTTP_IF_MODIFIED_SINCE set to the empty string. 

Caught this while profiling using rack-mini-profiler

```
Exceptions (2 raised during request)

ArgumentError not RFC 2822 compliant date: ""
/usr/local/rvm/rubies/ruby-2.0.0-p0-turbo/lib/ruby/2.0.0/time.rb:456:in `rfc2822'
/var/www/discourse/vendor/bundle/ruby/2.0.0/gems/rack-1.4.5/lib/rack/conditionalget.rb:64:in `to_rfc2822'
/var/www/discourse/vendor/bundle/ruby/2.0.0/gems/rack-1.4.5/lib/rack/conditionalget.rb:48:in `fresh?'
/var/www/discourse/vendor/bundle/ruby/2.0.0/gems/rack-1.4.5/lib/rack/conditionalget.rb:27:in `call'
```