Ensure slurp keys are symbolized

If the parameter's key is not a symbol the path cannot be built.

Ex.
```ruby
class User < JsonApiClient::Resource
  belongs_to :organization
end 

user = User.create("id" => 12, "organization_id" => 10, "name" => "Gob Bluth")
#=> Exception, "Not all prefix parameters specified"

user = User.create(params.require(:user).permit(:id, :organization_id, :name))
#=> Exception, "Not all prefix parameters specified"
```

The exception is thrown on [line 35](https://github.com/jamesdphillips/json_api_client/blob/dc6e08313ec4cfbfdfd94486ae86f84bc7c1f087/lib/json_api_client/helpers/associable.rb#L35), in the example it equates to:
```"/organizations/%{id}/users" % { "organization_id" => 12 } #=> KeyError: key{organization_id} not found```