Remove references to NSError (#23)

- [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](../CONTRIBUTING.md).

This pull request includes (pick all that apply):

- [ ] Bugfixes
- [ ] New features
- [x] Breaking changes
- [ ] Documentation updates
- [ ] Unit tests
- [ ] Other

### Summary
- Remove references to `NSError`, use `Error` instead.

### Implementation
- Remove unnecessary casting to `NSError`.

### Test Plan
- Run `rake test`

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## Master
 
 #### Breaking
+- Remove unnecessary casting to `NSError` on `SessionTaskCompletion`.
 - `XMLNode.children` is no longer optional, defaults to empty array.
 
 #### Enhancements

Sources/Conduit/Networking/URLSessionClient.swift

@@ -8,7 +8,7 @@
 
 import Foundation
 
-public typealias SessionTaskCompletion = (Data?, URLResponse?, NSError?) -> Void
+public typealias SessionTaskCompletion = (Data?, URLResponse?, Error?) -> Void
 public typealias SessionTaskProgressHandler = (Progress) -> Void
 
 fileprivate typealias SessionCompletionHandler = (URLSession.AuthChallengeDisposition, URLCredential?) -> Void
@@ -145,7 +145,7 @@ public struct URLSessionClient: URLSessionClientType {
                 taskUploadProgresses[task.taskIdentifier] = nil
             }
 
-            completionHandler?(taskResponse.data, taskResponse.response, taskResponse.error as NSError?)
+            completionHandler?(taskResponse.data, taskResponse.response, taskResponse.error)
         }
 
         func registerCompletionHandler(taskIdentifier: Int,
@@ -284,7 +284,7 @@ public struct URLSessionClient: URLSessionClientType {
             switch middlewareProcessingResult {
             case .error(let error):
                 self.urlSession.delegateQueue.addOperation {
-                    completion(nil, nil, error as NSError)
+                    completion(nil, nil, error)
                 }
                 return
             case .value(let request):
@@ -366,7 +366,7 @@ public struct URLSessionClient: URLSessionClientType {
             // If for some reason the client isn't retained elsewhere, it will at least stay alive
             // while active tasks are running
             self.activeTaskQueueDispatchGroup.leave()
-            completion(data, response, error as NSError?)
+            completion(data, response, error)
         }
 
         return dataTask