Use var instead of let for Endpoint.Response properties

[emma-k-alexandra]

Currently I think the usage of let is a bit onerous in the data coming back from the API. I think it's fine if they're moved to vars, and could make some development a bit easier.

There currently isn't a great mechanism to modify the data you receive from the API without creating a brand new struct, which can be tedious. If your project uses the data structures provided by this package, and it needs to modify the data it gets in some way, I think it should be able to do that.