mod_http_oauth2: Add Cache-Control and Pragma headers per by RFC 6749 These are mostly for the various Client-facing endpoints, so the chance of browsers being involved is slightly lower than with the User-facing authorization endpoint, which already sent the Cache-Control header. Thanks to OAuch for pointing out.

This module lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_whitelist",

}
s2s_whitelist = {
    "example.org",
}
```