Cookie path filtering used `request_path.starts_with(cookie_path)`, so a
Path=/admin cookie was sent to sibling paths that merely share the string
prefix, such as /administrator or /admin-backup. Per RFC 6265 5.1.4 a prefix
match also requires the boundary to fall on a '/'.
Add a path_matches helper (identical path, or a prefix ending in '/', or a
prefix followed by '/') and use it in get_cookie_header and
get_js_visible_cookies.
Fixes#382.