I know this is a little late, but it could be useful
before you send your wp_mail call add a filter to wp_mail_content_type
add_filter('wp_mail_content_type','set_contenttype');
then in your set_contenttype function have it return the appropriate content type, in this case ‘text/html’
function set_contenttype($content_type){
return 'text/html';
}
works for me
