Introduce wpcf7_enctype_value(). Enctype must be application/x-www-form-urlencoded, multipart/form-data, text/plain or empty. Ref: http://www.w3.org/TR/html5/forms.html#attr-fs-enctype

git-svn-id: https://plugins.svn.wordpress.org/contact-form-7/trunk@928276 b8457f37-d9ea-0310-8a92-e5e31aec5664