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 not
used in cache keys.

WIP

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