fix($resource): don't allow using promises as `timeout` and log a warning

Promises never worked correctly as values for `timeout` in `$resource`, because the same value has
to be re-used for multiple requests (and it is not possible to `angular.copy()` a promise).
Now (in addition to ignoring a non-numeric `timeout`), a warning is logged to the console using
`$log.debug()`.

Partly fixes #13393.

BREAKING CHANGE:

Possible breaking change for users who updated their code to provide a `timeout`
promise for a `$resource` request in version 1.4.8.

Up to v1.4.7 (included), using a promise as a timeout in `$resource`, would silently
fail (i.e. have no effect).

In v1.4.8, using a promise as timeout would have the (buggy) behaviour described
in https://github.com/angular/angular.js/pull/12657#issuecomment-152108887
(i.e. it will work as expected for the first time you resolve the promise and will
cancel all subsequent requests after that - one has to re-create the resource
class. This is feature was not documented.)

With this change, using a promise as timeout in 1.4.9 onwsards is not allowed.
It will log a warning and ignore the timeout value.

If you need support for cancellable `$resource` actions, you should upgrade to
version 1.5 or higher.