Only one header of type can be cached

[dimadin]

When there are two or more headers of the same type on the page, only one is stored in the cache. This is because PHP stores multiple headers of the same type while WP Spider Cache assumes there is only one per type (like what server returns).

For example, by default, post's page has two Link headers

Array
(
    [0] => Vary: Cookie
    [1] => Content-Type: text/html; charset=UTF-8
    [2] => X-Pingback: http://example.com/xmlrpc.php
    [3] => Link: <http://example.com/wp-json/>; rel="https://api.w.org/"
    [4] => Link: <http://example.com/?p=1>; rel=shortlink
)

but only last can be saved

foreach ( headers_list() as $header ) {
    list( $k, $v ) = array_map( 'trim', explode( ':', $header, 2 ) );
    $this->cache['headers'][ $k ] = $v;
}

[JJJ]

Good catch! I'll merge your PR and make sure PHP 7.1 doesn't complain (about assuming the array exists.)