ensure that both prefix/postfix of / char in url will work

the prefix / when constructing the URL using URLByAppendingPathComponent should be removed cause in case the user added a trailer / the output is a double // that leads to 404.
Removing the prefix / when calling the method, results in both cases work regardless if the user passed a url with trailer / or not.

[note]
this is not an issue with the obj-c sdk since already we have already [take care of it](https://github.com/aerogear/aerogear-ios-push/blob/master/push-sdk/AGDeviceRegistration.m#L71)