[openstack|storage] Fix extracted request

`Fog::OpenStack::Storage#get_object_http_url` was extracted in
https://github.com/fog/fog/commit/8012318d but did not get the
modules/classes to put it back into the structure.

So it had become a private method and failing some tests.
Merge pull request #2773 from fog/openstack_fix_method_visibility

[openstack|storage] Fix extracted request