Correct the return value of the `pre_http_request` filter

When a request is performed with the HTTP API, the return value is expected to be [an array or a WP_Error](https://github.com/WordPress/WordPress/blob/3.9.1/wp-includes/class-http.php#L65).

When Airplane Mode short-circuits the request using the `pre_http_request` filter, it return boolean true. It should return a `WP_Error` instead.

In addition, the filter should return the default value passed to it if Airplane Mode isn't enabled. This prevents it from stomping anything else that is using the same filter.