Clear headers

After upgrading to pyrax 1.9.3 the `HEADERS` option stopped working. [This comment](https://github.com/rackspace/pyrax/issues/364#issuecomment-58927242) suggests you (now) need to use `clear=True` to ensure you are not sending headers that will cause the server to ignore your request. A [full description](https://github.com/rackspace/pyrax/issues/364#issuecomment-59079386) of the problem can also be found there.