[rackspace | queues] Use case-insensitive header access for Location.

I just hit an issue regarding a change in header case in the Cloud Queues `/claims` endpoint. Specifically, the header that used to be "Location" is now "location", so `claim#create` can no longer extract the claim's ID from the response:

```
undefined method `split' for nil:NilClass
/home/ash/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/fog-1.19.0/lib/fog/rackspace/models/queues/claim.rb:112:in `create'
/home/ash/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/fog-1.19.0/lib/fog/rackspace/models/queues/claim.rb:46:in `save'
/home/ash/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/fog-1.19.0/lib/fog/rackspace/models/queues/claims.rb:36:in `create'
```

```ruby
[2] pry(#<Fog::Rackspace::Queues::Claim>)> response.headers
=> {"content-type"=>"application/json; charset=utf-8",
 "location"=>"/v1/queues/peril_events/claims/52ebe1463ac24e595a263f25",
 "Connection"=>"close",
 "X-Project-Id"=>"629538",
 "content-length"=>"2550"}
```

/cc @krames @elight