Some lang polish

Just had a brief look at the whole library. Great work! I learned quite a lot from it, thank you @mattt

I did some polish for it, if you'd like to have a look:

* Change some `var` to `let`.
* Change `let data = xxx; if data {}` to `if let data = xxx {}`. In other part of the lib, optional binding is being used. It would be better to use it here as well.
* Fix a space
* Add `Alamofire.` to the connivence `request` method, making it seems like others.