Separate ViewContext get/create context behavior

Separating these behaviors eliminates and entire class of bugs like forgetting to re-set the thread local, attempting to re-set the block-local conditionally, and so on. It also makes it more obvious that `self.current` is memoizing.

lib/draper/view_context.rb

@@ -1,13 +1,7 @@
 module Draper
   module ViewContext
     def self.current
-      context = Thread.current[:current_view_context]
-      context ||= ApplicationController.new.view_context
-      context.controller.request ||= ActionController::TestRequest.new
-      context.request            ||= context.controller.request
-      context.params             ||= {}
-      Thread.current[:current_view_context] = context
-      context
+      Thread.current[:current_view_context] ||= build_view_context
     end
 
     def self.current=(input)
@@ -19,5 +13,15 @@ def view_context
         Draper::ViewContext.current = context
       end
     end
+
+    private
+
+    def build_view_context
+      ApplicationController.new.view_context.tap do |context|
+        context.controller.request ||= ActionController::TestRequest.new
+        context.request            ||= context.controller.request
+        context.params             ||= {}
+      end
+    end
   end
 end