fix session warning message

git-svn-id: https://plugins.svn.wordpress.org/contact-form-7-radio-field-mandatory/trunk@1044727 b8457f37-d9ea-0310-8a92-e5e31aec5664
fixed session header warning

git-svn-id: https://plugins.svn.wordpress.org/contact-form-7-radio-field-mandatory/trunk@1044735 b8457f37-d9ea-0310-8a92-e5e31aec5664