Fix performance regression with empty template rendering

Closes rspec#1537.

The change in 80637fc introduced a performance regression because the
template lookup cache was being lost every time an example was run.

This commit fixes the problem by caching the EmptyTemplateResolver
instances between example runs in a hash indexed by the path string.

Author: pixeltrix

lib/rspec/rails/view_rendering.rb

@@ -85,16 +85,18 @@ def _path_decorator(path)
         end
       end
 
+      # @private
+      RESOLVER_CACHE = Hash.new do |hash, path|
+        hash[path] = EmptyTemplateResolver.new(path)
+      end
+
       included do
         before do
           unless render_views?
             @_original_path_set = controller.class.view_paths
+            path_set = @_original_path_set.map { |resolver| RESOLVER_CACHE[resolver.to_s] }
 
-            empty_template_path_set = @_original_path_set.map do |resolver|
-              EmptyTemplateResolver.new(resolver.to_s)
-            end
-
-            controller.class.view_paths = empty_template_path_set
+            controller.class.view_paths = path_set
             controller.extend(EmptyTemplates)
           end
         end