WordPress has wp_kses function to makes sure that only the allowed HTML element names, attribute names and attribute values plus only sane HTML entities will occur in $string. You have to remove any slashes from PHP’s magic quotes before you call this function.

For your purpose you can try remove this filter by adding this code within your theme functions

    function kses_remove_filters() {
    // Normal filtering
    remove_filter('title_save_pre', 'wp_filter_kses');

    // Comment filtering
    remove_filter( 'pre_comment_content', 'wp_filter_post_kses' );
    remove_filter( 'pre_comment_content', 'wp_filter_kses' );

    // Post filtering
    remove_filter('content_save_pre', 'wp_filter_post_kses');
    remove_filter('excerpt_save_pre', 'wp_filter_post_kses');
    remove_filter('content_filtered_save_pre', 'wp_filter_post_kses');
}

or just

    kses_remove_filters();

Hope it solves your problem