Ensure format is in the type that rails expects

Rails expects the format to be a symbol. It filters formats against
`Mime::SET.symbols` so formats which are unknown (or are known but are
of an unexpected type) are not used in a cache keys for the Rails'
`ActionView::Resolver::Cache`. This made the view cache unusable when
there was multiple templates with the same name but different format
and was the the reason the cache was disabled in [1].

[1] rspec#864

Author: zetter

lib/rspec/rails/example/view_example_group.rb

@@ -114,7 +114,7 @@ def _default_render_options
 
             render_options = { :template => template }
             render_options[:handlers] = [handler] if handler
-            render_options[:formats] = [format] if format
+            render_options[:formats] = [format.to_sym] if format
             render_options[:locales] = [locale] if locale
 
             render_options

spec/rspec/rails/example/view_example_group_spec.rb

@@ -121,7 +121,7 @@ def _assigns
           view_spec.render
 
           if ::Rails::VERSION::STRING >= '3.2'
-            expect(view_spec.received.first).to eq([{:template => "widgets/new", :locales=>['en'], :formats=>['html'], :handlers=>['erb']}, {}, nil])
+            expect(view_spec.received.first).to eq([{:template => "widgets/new", :locales=>['en'], :formats=>[:html], :handlers=>['erb']}, {}, nil])
           else
             expect(view_spec.received.first).to eq([{:template => "widgets/new.en.html.erb"}, {}, nil])
           end