Fog::Storage::OpenStack::Real#put_object: Don't use data when block is provided

Making request with 'Content-Length: 0' fails with status 499
Fog::Storage::OpenStack::Real#put_object: Document the block parameter
Fog::Storage::Rackspace::Real#put_object: Same
Merge pull request #2165 from dgutov/open_stack_put_object_with_block

Fog::Storage::OpenStack::Real#put_object: Don't use data when block is provided