Fix ActionView::Template#formats issue in Rails 6

In Rails 6 the `ActionView::Template#formats` method has been deprecated
[1] and replaced with `ActionView::Template#format` method.

I have attempted to retain support for Rails <= 5.2.

[1] https://www.github.com/rails/rails/commit/ca5e23ed4d8

lib/rspec/rails/view_rendering.rb

@@ -56,11 +56,21 @@ def self.nullify_template_rendering(templates)
               template.identifier,
               EmptyTemplateHandler,
               :virtual_path => template.virtual_path,
-              :format => template.formats
+              :format => template_format(template)
             )
           end
         end
 
+        if ::Rails::VERSION::STRING >= '6'
+          def self.template_format(template)
+            template.format
+          end
+        else
+          def self.template_format(template)
+            template.formats
+          end
+        end
+
         # Delegates all methods to the submitted resolver and for all methods
         # that return a collection of `ActionView::Template` instances, return
         # templates with modified source