Set view_context if it's nil.

This should take care of #244, but also in general, anywhere
that we haven't primed the context yet.

lib/draper/view_context.rb

@@ -1,7 +1,7 @@
 module Draper
   module ViewContext
     def self.current
-      Thread.current[:current_view_context]
+      Thread.current[:current_view_context] || view_context
     end
 
     def self.current=(input)